cssforge_core/

engine.rs

use crate::{
    model::{
        AnalysisStats, FileReport, Finding, PlanEntry, Proof, RuleId, Safety, SourceRange,
        WorkspaceReport, WorkspaceSummary,
    },
    scanner::{
        NodeKind, SourceNode, count_ascii_case_insensitive_outside_comments,
        count_top_level_declarations, is_whitespace_only, scan_nodes, top_level_declarations,
    },
};
use anyhow::{Context, Result};
use lightningcss::stylesheet::{ParserOptions, StyleSheet};
use similar::TextDiff;
use std::{
    collections::{HashMap, HashSet},
    fs,
    ops::Range,
    path::{Path, PathBuf},
};

const SPEC_BASELINE: &str = "2026-08-17";

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Default)]
pub struct Specificity {
    pub ids: usize,
    pub classes: usize,
    pub elements: usize,
}

pub fn calculate_specificity(selector: &str) -> Specificity {
    let mut ids = 0;
    let mut classes = 0;
    let mut elements = 0;
    let bytes = selector.as_bytes();
    let mut i = 0;
    let mut in_attr = false;

    while i < bytes.len() {
        let b = bytes[i];
        if b == b'[' {
            in_attr = true;
            classes += 1;
            i += 1;
            continue;
        }
        if b == b']' {
            in_attr = false;
            i += 1;
            continue;
        }
        if in_attr {
            i += 1;
            continue;
        }

        if b == b'#' {
            ids += 1;
            i += 1;
            while i < bytes.len()
                && (bytes[i].is_ascii_alphanumeric() || bytes[i] == b'-' || bytes[i] == b'_')
            {
                i += 1;
            }
            continue;
        }

        if b == b'.' {
            classes += 1;
            i += 1;
            while i < bytes.len()
                && (bytes[i].is_ascii_alphanumeric() || bytes[i] == b'-' || bytes[i] == b'_')
            {
                i += 1;
            }
            continue;
        }

        if b == b':' {
            if i + 1 < bytes.len() && bytes[i + 1] == b':' {
                elements += 1;
                i += 2;
                while i < bytes.len()
                    && (bytes[i].is_ascii_alphanumeric() || bytes[i] == b'-' || bytes[i] == b'_')
                {
                    i += 1;
                }
            } else {
                let start_name = i + 1;
                let mut end_name = start_name;
                while end_name < bytes.len()
                    && (bytes[end_name].is_ascii_alphanumeric() || bytes[end_name] == b'-')
                {
                    end_name += 1;
                }
                let pseudo_name = &selector[start_name..end_name];
                if pseudo_name == "where" {
                    if end_name < bytes.len()
                        && bytes[end_name] == b'('
                        && let Some(close_p) = find_matching_paren(selector, end_name)
                    {
                        i = close_p + 1;
                        continue;
                    }
                } else if pseudo_name == "is" || pseudo_name == "not" || pseudo_name == "has" {
                    if end_name < bytes.len()
                        && bytes[end_name] == b'('
                        && let Some(close_p) = find_matching_paren(selector, end_name)
                    {
                        let inner = &selector[end_name + 1..close_p];
                        let max_inner = split_top_level_comma(inner)
                            .into_iter()
                            .map(|s| calculate_specificity(s.trim()))
                            .max()
                            .unwrap_or_default();
                        ids += max_inner.ids;
                        classes += max_inner.classes;
                        elements += max_inner.elements;
                        i = close_p + 1;
                        continue;
                    }
                    classes += 1;
                } else {
                    classes += 1;
                }
                i = end_name;
            }
            continue;
        }

        if (b.is_ascii_alphabetic() || b == b'*')
            && (i == 0
                || bytes[i - 1].is_ascii_whitespace()
                || bytes[i - 1] == b'>'
                || bytes[i - 1] == b'+'
                || bytes[i - 1] == b'~'
                || bytes[i - 1] == b'|')
        {
            if b != b'*' {
                elements += 1;
            }
            i += 1;
            while i < bytes.len()
                && (bytes[i].is_ascii_alphanumeric() || bytes[i] == b'-' || bytes[i] == b'_')
            {
                i += 1;
            }
            continue;
        }

        i += 1;
    }

    Specificity {
        ids,
        classes,
        elements,
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum RelationKind {
    PseudoClass,
    PseudoElement,
    Attribute,
    Compound,
    Descendant,
    Combinator,
}

impl RelationKind {
    fn rule(self) -> RuleId {
        match self {
            Self::PseudoClass => RuleId::NestPseudoClass,
            Self::PseudoElement => RuleId::NestPseudoElement,
            Self::Attribute => RuleId::NestAttribute,
            Self::Compound => RuleId::NestCompound,
            Self::Descendant => RuleId::NestDescendant,
            Self::Combinator => RuleId::NestCombinator,
        }
    }
}

#[derive(Debug, Clone)]
enum ConditionalInner {
    Direct {
        body_range: Range<usize>,
    },
    Nested {
        nested_selector: String,
        body_range: Range<usize>,
    },
}

#[derive(Debug, Clone)]
enum ClusterChild {
    Style {
        node: SourceNode,
        relation: RelationKind,
        nested_selector: String,
    },
    Conditional {
        node: SourceNode,
        rule: RuleId,
        inners: Vec<ConditionalInner>,
    },
}

impl ClusterChild {
    fn node(&self) -> &SourceNode {
        match self {
            Self::Style { node, .. } | Self::Conditional { node, .. } => node,
        }
    }

    fn rule(&self) -> RuleId {
        match self {
            Self::Style { relation, .. } => relation.rule(),
            Self::Conditional { rule, .. } => *rule,
        }
    }
}

pub fn analyze_file(path: &Path, enabled_rules: &[RuleId]) -> Result<FileReport> {
    let source = fs::read_to_string(path)
        .with_context(|| format!("failed to read CSS file {}", path.display()))?;
    analyze_content(path.to_path_buf(), &source, enabled_rules)
}

/// Return the content ranges of HTML/template `<style>` elements. The tag
/// itself is intentionally excluded so transformations can be spliced back
/// without changing Blade/PHP/HTML markup.
pub fn extract_style_blocks(source: &str) -> Vec<Range<usize>> {
    let lower = source.to_ascii_lowercase();
    let bytes = lower.as_bytes();
    let mut blocks = Vec::new();
    let mut cursor = 0usize;
    while let Some(relative) = lower[cursor..].find("<style") {
        let open = cursor + relative;
        let after_name = open + "<style".len();
        if bytes
            .get(after_name)
            .is_none_or(|b| !b.is_ascii_whitespace() && *b != b'>')
        {
            cursor = after_name;
            continue;
        }
        let Some(open_end_rel) = lower[after_name..].find('>') else {
            break;
        };
        let content_start = after_name + open_end_rel + 1;
        let Some(close_rel) = lower[content_start..].find("</style") else {
            break;
        };
        let close_start = content_start + close_rel;
        let Some(close_end_rel) = lower[close_start..].find('>') else {
            break;
        };
        blocks.push(content_start..close_start);
        cursor = close_start + close_end_rel + 1;
    }
    blocks
}

fn analyze_content(path: PathBuf, source: &str, enabled_rules: &[RuleId]) -> Result<FileReport> {
    let blocks = extract_style_blocks(source);
    if blocks.is_empty() {
        return analyze_source(path, source, enabled_rules);
    }

    let mut report = FileReport {
        path,
        parse_ok: true,
        parse_error: None,
        stats: AnalysisStats::default(),
        findings: Vec::new(),
        plans: Vec::new(),
    };
    report.stats.bytes = source.len();

    for (index, range) in blocks.iter().enumerate() {
        let mut embedded =
            analyze_source(report.path.clone(), &source[range.clone()], enabled_rules)?;
        if !embedded.parse_ok {
            report.parse_ok = false;
            if report.parse_error.is_none() {
                report.parse_error = embedded
                    .parse_error
                    .take()
                    .map(|error| format!("style block {}: {error}", index + 1));
            }
        }
        report.stats.top_level_style_rules += embedded.stats.top_level_style_rules;
        report.stats.top_level_at_rules += embedded.stats.top_level_at_rules;
        report.stats.declarations += embedded.stats.declarations;
        report.stats.important_declarations += embedded.stats.important_declarations;
        report.stats.custom_properties += embedded.stats.custom_properties;
        report.stats.duplicate_selectors += embedded.stats.duplicate_selectors;
        report.stats.media_rules += embedded.stats.media_rules;
        report.stats.supports_rules += embedded.stats.supports_rules;
        report.stats.container_rules += embedded.stats.container_rules;
        report.stats.layer_rules += embedded.stats.layer_rules;
        report.stats.scope_rules += embedded.stats.scope_rules;
        report.stats.starting_style_rules += embedded.stats.starting_style_rules;
        report.stats.parse_errors += embedded.stats.parse_errors;
        report.findings.append(&mut embedded.findings);
        for mut plan in embedded.plans {
            plan.source_range.start += range.start;
            plan.source_range.end += range.start;
            plan.original = source[plan.source_range.start..plan.source_range.end].to_string();
            report.plans.push(plan);
        }
    }
    Ok(report)
}

pub fn analyze_workspace(
    root: &Path,
    files: &[PathBuf],
    enabled_rules: &[RuleId],
) -> Result<WorkspaceReport> {
    let mut reports = Vec::with_capacity(files.len());
    let mut next_id = 1usize;

    for path in files {
        let mut report = analyze_file(path, enabled_rules)?;
        for plan in &mut report.plans {
            plan.id = format!("T-{next_id:06}");
            next_id += 1;
        }
        reports.push(report);
    }

    let mut summary = WorkspaceSummary {
        files: reports.len(),
        ..WorkspaceSummary::default()
    };

    for report in &reports {
        if !report.parse_ok {
            summary.parse_errors += 1;
        }
        summary.rules_analyzed +=
            report.stats.top_level_style_rules + report.stats.top_level_at_rules;
        for plan in &report.plans {
            match plan.safety {
                Safety::Safe => summary.safe += 1,
                Safety::Review => summary.review += 1,
                Safety::Unsafe => summary.unsafe_count += 1,
                Safety::Unsupported => summary.unsupported += 1,
                Safety::NoOp => summary.no_op += 1,
            }
            if plan
                .warnings
                .iter()
                .any(|w| w.to_ascii_lowercase().contains("specificity"))
            {
                summary.specificity_sensitive += 1;
            }
            if plan.warnings.iter().any(|w| {
                w.to_ascii_lowercase().contains("cascade")
                    || w.to_ascii_lowercase().contains("source order")
            }) {
                summary.cascade_sensitive += 1;
            }
            if plan
                .warnings
                .iter()
                .any(|w| w.to_ascii_lowercase().contains("layer"))
            {
                summary.layer_sensitive += 1;
            }
            if plan
                .warnings
                .iter()
                .any(|w| w.to_ascii_lowercase().contains("scope"))
            {
                summary.scope_sensitive += 1;
            }
        }
    }

    Ok(WorkspaceReport {
        tool_version: env!("CARGO_PKG_VERSION").to_string(),
        spec_baseline: SPEC_BASELINE.to_string(),
        root: root.to_path_buf(),
        enabled_rules: enabled_rules.to_vec(),
        files: reports,
        summary,
    })
}

fn analyze_source(path: PathBuf, source: &str, enabled_rules: &[RuleId]) -> Result<FileReport> {
    // LightningCSS is a *validator*, not the planner. Unknown constructs
    // (`::picker()`, `::checkmark`, `anchor-size()`) must not wipe plans:
    // `scan_nodes` already sees those as ordinary style/at-rule blocks.
    let parse_result = StyleSheet::parse(
        source,
        ParserOptions {
            filename: path.display().to_string(),
            error_recovery: true,
            ..ParserOptions::default()
        },
    );

    let parse_error = parse_result.err().map(|err| format!("{err:?}"));
    let parse_ok = parse_error.is_none();
    let nodes = scan_nodes(source, 0..source.len());
    let stats = collect_stats(source, &nodes, parse_ok);
    let mut findings = collect_findings(source, &nodes);

    if let Some(error) = &parse_error {
        findings.push(Finding {
            safety: Safety::Unsupported,
            title: "Semantic parse failed".into(),
            detail: format!(
                "{error} Planning continues from the structural scanner; review all plans."
            ),
        });
    }

    let mut plans = if !enabled_rules.is_empty() {
        build_plans_recursive(&path, source, &nodes, enabled_rules)
    } else {
        Vec::new()
    };
    if !parse_ok {
        for plan in &mut plans {
            if plan.safety == Safety::Safe {
                plan.safety = Safety::Review;
            }
            plan.warnings.push(
                "Planned despite a LightningCSS parse error on this file; review required.".into(),
            );
        }
    }

    Ok(FileReport {
        path,
        parse_ok,
        parse_error,
        stats,
        findings,
        plans,
    })
}

fn collect_stats(source: &str, nodes: &[SourceNode], parse_ok: bool) -> AnalysisStats {
    let mut stats = AnalysisStats {
        bytes: source.len(),
        parse_errors: usize::from(!parse_ok),
        important_declarations: count_ascii_case_insensitive_outside_comments(source, "!important"),
        ..AnalysisStats::default()
    };
    let mut selector_counts: HashMap<String, usize> = HashMap::new();

    for node in nodes {
        match &node.kind {
            NodeKind::Style => {
                stats.top_level_style_rules += 1;
                let selector = node.prelude(source).to_string();
                *selector_counts.entry(selector).or_default() += 1;
                if let Some(body) = node.body(source) {
                    stats.declarations += count_top_level_declarations(body);
                    stats.custom_properties += count_custom_properties(body);
                }
            }
            NodeKind::AtBlock { name, .. } => {
                stats.top_level_at_rules += 1;
                match name.as_str() {
                    "media" => stats.media_rules += 1,
                    "supports" => stats.supports_rules += 1,
                    "container" => stats.container_rules += 1,
                    "layer" => stats.layer_rules += 1,
                    "scope" => stats.scope_rules += 1,
                    "starting-style" => stats.starting_style_rules += 1,
                    _ => {}
                }
            }
            NodeKind::AtStatement { .. } => stats.top_level_at_rules += 1,
        }
    }

    stats.duplicate_selectors = selector_counts.values().filter(|&&count| count > 1).count();
    stats
}

fn count_custom_properties(body: &str) -> usize {
    body.lines()
        .filter(|line| {
            let trimmed = line.trim_start();
            trimmed.starts_with("--") && trimmed.contains(':')
        })
        .count()
}

fn collect_findings(source: &str, nodes: &[SourceNode]) -> Vec<Finding> {
    let mut findings = Vec::new();
    let selectors: HashSet<String> = nodes
        .iter()
        .filter(|n| matches!(&n.kind, NodeKind::Style))
        .map(|n| n.prelude(source).to_string())
        .collect();

    let mut selector_occurrences: HashMap<String, usize> = HashMap::new();

    for node in nodes {
        match &node.kind {
            NodeKind::Style => {
                let selector = node.prelude(source);
                *selector_occurrences.entry(selector.to_string()).or_default() += 1;

                if contains_top_level_comma(selector) {
                    let branches = split_top_level_comma(selector);
                    let specs: Vec<Specificity> = branches.iter().map(|b| calculate_specificity(b.trim())).collect();
                    let has_mixed = specs.windows(2).any(|w| w[0] != w[1]);
                    if has_mixed {
                        findings.push(Finding {
                            safety: Safety::Review,
                            title: "Mixed-specificity selector list detected".into(),
                            detail: format!("{selector}: contains branches with differing specificities; factoring into :is() or parent nesting would raise lower-specificity branches."),
                        });
                    } else {
                        findings.push(Finding {
                            safety: Safety::Review,
                            title: "Selector list kept flat".into(),
                            detail: format!("{selector}: parent selector lists require per-branch specificity proof before native nesting."),
                        });
                    }
                }

                if let Some(base) = bem_base_candidate(selector)
                    && selectors.contains(base) {
                        findings.push(Finding {
                            safety: Safety::Unsupported,
                            title: "BEM token concatenation is not native nesting".into(),
                            detail: format!("{selector} resembles {base} + a BEM suffix; CSS nesting cannot safely generate &__element or &--modifier."),
                        });
                    }

                if let Some(body) = node.body(source) {
                    if body.trim().is_empty() {
                        findings.push(Finding {
                            safety: Safety::Review,
                            title: "Empty rule block detected".into(),
                            detail: format!("{selector} contains no declarations or nested rules."),
                        });
                    }

                    let mut seen_props: HashMap<String, String> = HashMap::new();
                    for (raw_prop, raw_val) in top_level_declarations(body) {
                        let prop = raw_prop.to_ascii_lowercase();
                        let val = raw_val.trim_end_matches(';').trim().to_string();
                        if let Some(prev_val) = seen_props.get(&prop) {
                            if prev_val == &val {
                                findings.push(Finding {
                                    safety: Safety::Review,
                                    title: "Exact duplicate declaration detected".into(),
                                    detail: format!("In {selector}: property '{prop}: {val}' is declared multiple times with identical value."),
                                });
                            }
                        } else {
                            seen_props.insert(prop, val);
                        }
                    }

                    if selector.contains(" .") && !selector.contains(":has(") {
                        findings.push(Finding {
                            safety: Safety::Review,
                            title: "Potential :has() relational candidate".into(),
                            detail: format!("{selector}: parent-child descendant relationship could be expressed with :has() if container-targeting is intended (advisory)."),
                        });
                    }
                }
            }
            NodeKind::AtBlock { name, .. } => match name.as_str() {
                "layer" => findings.push(Finding {
                    safety: Safety::Review,
                    title: "Cascade layer context detected".into(),
                    detail: "@layer participates in cascade ordering and reverses layer precedence for !important; automatic layer architecture is not applied.".into(),
                }),
                "scope" => findings.push(Finding {
                    safety: Safety::Review,
                    title: "Scope boundary detected".into(),
                    detail: "@scope boundaries enforce doughnut scoping; scoping parameters require manual architect review.".into(),
                }),
                "container" => findings.push(Finding {
                    safety: Safety::Review,
                    title: "Container query context detected".into(),
                    detail: "@container depends on eligible ancestor containers; media-to-container conversion is not inferred from CSS alone.".into(),
                }),
                "starting-style" => findings.push(Finding {
                    safety: Safety::Review,
                    title: "Starting-style context detected".into(),
                    detail: "@starting-style is temporal transition state; this build never invents it from ordinary declarations.".into(),
                }),
                _ => {}
            },
            NodeKind::AtStatement { .. } => {}
        }
    }

    for (selector, count) in selector_occurrences {
        if count > 1 {
            findings.push(Finding {
                safety: Safety::Review,
                title: "Duplicate selector in stylesheet".into(),
                detail: format!("'{selector}' appears {count} times in the stylesheet; non-adjacent occurrences must not be merged across intervening rules."),
            });
        }
    }

    findings
}

fn bem_base_candidate(selector: &str) -> Option<&str> {
    if let Some(pos) = selector.find("__") {
        let base = &selector[..pos];
        if !base.is_empty() && !base.contains(' ') {
            return Some(base);
        }
    }
    if let Some(pos) = selector.find("--") {
        let base = &selector[..pos];
        if !base.is_empty() && !base.contains(' ') {
            return Some(base);
        }
    }
    None
}

// These at-rules keep the nearest style-rule nesting context while their
// contents are parsed.  `@starting-style` is a conditional group rule too;
// omitting it here means nested selectors inside it are never planned.
const TRANSPARENT_AT_RULES: &[&str] = &[
    "layer",
    "scope",
    "media",
    "supports",
    "container",
    "starting-style",
];

fn build_plans_recursive(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled_rules: &[RuleId],
) -> Vec<PlanEntry> {
    let mut plans = build_plans(path, source, nodes, enabled_rules);

    for node in nodes {
        let should_recurse = match &node.kind {
            NodeKind::AtBlock { name, .. } => TRANSPARENT_AT_RULES.contains(&name.as_str()),
            // Native CSS nesting puts child style rules inside a style rule's
            // body. They need the same planning pass as top-level rules.
            NodeKind::Style => true,
            NodeKind::AtStatement { .. } => false,
        };

        if should_recurse {
            let is_covered = plans
                .iter()
                .any(|p| p.source_range.start <= node.start && node.end <= p.source_range.end);
            if !is_covered && let Some(body_range) = &node.body_range {
                let inner_nodes = scan_nodes(source, body_range.clone());
                if !inner_nodes.is_empty() {
                    let inner_plans =
                        build_plans_recursive(path, source, &inner_nodes, enabled_rules);
                    plans.extend(inner_plans);
                }
            }
        }
    }

    let keep = select_disjoint_plan_indices(&plans);
    keep.into_iter().map(|i| plans[i].clone()).collect()
}

fn build_plans(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled_rules: &[RuleId],
) -> Vec<PlanEntry> {
    let enabled: HashSet<RuleId> = enabled_rules.iter().copied().collect();
    let mut plans = Vec::new();

    // 1. Structural At-rule refactorings across top-level nodes
    plan_merge_same_named_layers(path, source, nodes, &enabled, &mut plans);
    plan_merge_adjacent_at_blocks(path, source, nodes, &enabled, &mut plans);
    plan_gather_consecutive_conditions_by_selector(path, source, nodes, &enabled, &mut plans);
    plan_merge_adjacent_identical_selectors(path, source, nodes, &enabled, &mut plans);
    plan_gather_related_selector_rules(path, source, nodes, &enabled, &mut plans);
    plan_nest_layer_by_selector(path, source, nodes, &enabled, &mut plans);
    plan_merge_identical_rule_bodies(path, source, nodes, &enabled, &mut plans);
    plan_factor_identical_states_with_is(path, source, nodes, &enabled, &mut plans);
    plan_factor_multi_selector_cluster_with_is(path, source, nodes, &enabled, &mut plans);
    plan_nest_in_place_adjacent_states(path, source, nodes, &enabled, &mut plans);

    let mut i = 0usize;

    while i < nodes.len() {
        let parent = &nodes[i];

        // ModernizeMediaRange on at-rules
        if enabled.contains(&RuleId::ModernizeMediaRange)
            && let NodeKind::AtBlock { name, .. } = &parent.kind
            && (name == "media" || name == "container")
        {
            let prelude = parent.prelude(source);
            if let Some(modernized) = modernize_media_query_str(prelude) {
                plans.push(PlanEntry {
                            id: String::new(),
                            file: path.to_path_buf(),
                            rules: vec![RuleId::ModernizeMediaRange],
                            safety: Safety::Safe,
                            source_range: SourceRange {
                                start: parent.prelude_range.start,
                                end: parent.prelude_range.end,
                            },
                            original: source[parent.prelude_range.clone()].to_string(),
                            proposed: modernized,
                            proof: Proof::safe_local(),
                            warnings: Vec::new(),
                            reason: "Modernize legacy media/container feature syntax to CSS Range Syntax (e.g. (width >= 800px)).".to_string(),
                            selected: true,
                        });
            }
        }

        if !matches!(&parent.kind, NodeKind::Style) {
            i += 1;
            continue;
        }

        let parent_selector = parent.prelude(source);

        // FactorSelectorList
        if contains_top_level_comma(parent_selector) {
            let parent_indent = line_indent(source, parent.start);
            let unit = relative_indent_unit(source, parent);
            let already_nested_selector_list = split_top_level_comma(parent_selector)
                .iter()
                .any(|branch| branch.trim().starts_with('&'));

            if enabled.contains(&RuleId::FactorSelectorList)
                && !already_nested_selector_list
                && let Some(body_range) = &parent.body_range
            {
                let body = &source[body_range.clone()];
                if let Some(mut factored) =
                    factor_selector_list(parent_selector, body, &parent_indent, &unit)
                {
                    let branches: Vec<&str> = split_top_level_comma(parent_selector)
                        .into_iter()
                        .map(|s| s.trim())
                        .collect();
                    let base = branches[0];

                    // Check if subsequent adjacent style rules share base (e.g. .notice:hover)
                    let mut cursor = i + 1;
                    let mut prev_end = parent.end;
                    let mut extra_children = Vec::new();

                    while cursor < nodes.len() {
                        let next = &nodes[cursor];
                        if !is_whitespace_only(source, prev_end..next.start) {
                            break;
                        }
                        if matches!(&next.kind, NodeKind::Style)
                            && let Some((rel, nested_sel)) =
                                selector_relation(base, next.prelude(source))
                            && enabled.contains(&rel.rule())
                        {
                            extra_children.push(ClusterChild::Style {
                                node: next.clone(),
                                relation: rel,
                                nested_selector: nested_sel,
                            });
                            prev_end = next.end;
                            cursor += 1;
                            continue;
                        }
                        break;
                    }

                    let end_offset = if extra_children.is_empty() {
                        parent.end
                    } else {
                        let nested_indent = format!("{parent_indent}{unit}");
                        let inner_decl_indent = format!("{nested_indent}{unit}");
                        let mut extra_rendered = String::new();

                        for ch in &extra_children {
                            if let ClusterChild::Style {
                                node: ch_node,
                                nested_selector,
                                ..
                            } = ch
                            {
                                extra_rendered.push('\n');
                                extra_rendered.push_str(&nested_indent);
                                extra_rendered.push_str(nested_selector.trim());
                                extra_rendered.push_str(" {\n");
                                if let Some(ch_body_range) = &ch_node.body_range {
                                    for line in source[ch_body_range.clone()].lines() {
                                        let trimmed = line.trim();
                                        if !trimmed.is_empty() {
                                            extra_rendered.push_str(&inner_decl_indent);
                                            extra_rendered.push_str(&ensure_semicolon(trimmed));
                                            extra_rendered.push('\n');
                                        }
                                    }
                                }
                                extra_rendered.push_str(&nested_indent);
                                extra_rendered.push_str("}\n");
                            }
                        }

                        if let Some(close_brace_pos) = factored.rfind('}') {
                            factored.insert_str(close_brace_pos, &extra_rendered);
                        }
                        prev_end
                    };

                    plans.push(PlanEntry {
                            id: String::new(),
                            file: path.to_path_buf(),
                            rules: vec![RuleId::FactorSelectorList],
                            safety: Safety::Safe,
                            source_range: SourceRange {
                                start: parent.start,
                                end: end_offset,
                            },
                            original: source[parent.start..end_offset].to_string(),
                            proposed: factored,
                            proof: Proof::safe_local(),
                            warnings: Vec::new(),
                            reason: "Factor comma-separated selectors sharing a common base element into nested form.".to_string(),
                            selected: true,
                        });
                    i = cursor;
                    continue;
                }
            }

            if enabled.contains(&RuleId::ModernizeIs)
                && let Some((factored_sel, uniform)) = factor_with_is(parent_selector)
            {
                plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::ModernizeIs],
                        safety: if uniform { Safety::Safe } else { Safety::Review },
                        source_range: SourceRange {
                            start: parent.prelude_range.start,
                            end: parent.prelude_range.end,
                        },
                        original: source[parent.prelude_range.clone()].to_string(),
                        proposed: factored_sel,
                        proof: Proof {
                            specificity_equivalent: uniform,
                            ..Proof::safe_local()
                        },
                        warnings: if uniform { Vec::new() } else { vec!["Mixed branch specificity: :is() takes the specificity of its most specific argument.".into()] },
                        reason: "Factor common selector prefix/suffix into :is(...) grouping.".to_string(),
                        selected: true,
                    });
                i += 1;
                continue;
            }

            if enabled.contains(&RuleId::ModernizeWhere)
                && let Some(factored_where) = factor_with_where(parent_selector)
            {
                plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::ModernizeWhere],
                        safety: Safety::Review,
                        source_range: SourceRange {
                            start: parent.prelude_range.start,
                            end: parent.prelude_range.end,
                        },
                        original: source[parent.prelude_range.clone()].to_string(),
                        proposed: factored_where,
                        proof: Proof {
                            specificity_equivalent: false,
                            ..Proof::safe_local()
                        },
                        warnings: vec!["Specificity zeroed to 0-0-0 by :where()".into()],
                        reason: "Convert selector list to :where(...) for zero-specificity defaults (review required).".to_string(),
                        selected: true,
                    });
                i += 1;
                continue;
            }

            i += 1;
            continue;
        }

        if parent_selector.contains("::") {
            i += 1;
            continue;
        }

        let mut children = Vec::new();
        let mut cursor = i + 1;
        let mut previous_end = parent.end;

        while cursor < nodes.len() {
            let node = &nodes[cursor];
            if !is_whitespace_only(source, previous_end..node.start) {
                break;
            }

            if matches!(&node.kind, NodeKind::Style)
                && let Some((relation, nested_selector)) =
                    selector_relation(parent_selector, node.prelude(source))
                && enabled.contains(&relation.rule())
            {
                children.push(ClusterChild::Style {
                    node: node.clone(),
                    relation,
                    nested_selector,
                });
                previous_end = node.end;
                cursor += 1;
                continue;
            }

            if let Some(child) = conditional_child(source, parent_selector, node, &enabled) {
                previous_end = node.end;
                children.push(child);
                cursor += 1;
                continue;
            }

            break;
        }

        if !children.is_empty() {
            let last_end = children.last().expect("non-empty cluster").node().end;
            let proposed = render_cluster(source, parent, &children);
            let mut rules = Vec::new();
            for child in &children {
                let rule = child.rule();
                if !rules.contains(&rule) {
                    rules.push(rule);
                }
            }
            plans.push(PlanEntry {
                id: String::new(),
                file: path.to_path_buf(),
                rules,
                safety: Safety::Safe,
                source_range: SourceRange {
                    start: parent.start,
                    end: last_end,
                },
                original: source[parent.start..last_end].to_string(),
                proposed,
                proof: Proof::safe_local(),
                warnings: Vec::new(),
                reason: format!(
                    "{} immediately adjacent rule(s) share the exact parent selector and can be nested without crossing comments or unrelated rules.",
                    children.len()
                ),
                selected: true,
            });
            i = cursor;
        } else {
            if enabled.contains(&RuleId::ConsolidateNot) && matches!(&parent.kind, NodeKind::Style)
            {
                let prelude = parent.prelude(source);
                if let Some((consolidated, _uniform)) = consolidate_not_in_selector(prelude) {
                    plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::ConsolidateNot],
                        safety: Safety::Review,
                        source_range: SourceRange {
                            start: parent.prelude_range.start,
                            end: parent.prelude_range.end,
                        },
                        original: source[parent.prelude_range.clone()].to_string(),
                        proposed: consolidated,
                        proof: Proof {
                            specificity_equivalent: false,
                            ..Proof::safe_local()
                        },
                        warnings: vec!["Specificity reduced: chained :not() has additive specificity; comma-separated :not() takes only the maximum argument specificity.".into()],
                        reason: "Consolidate chained :not() selectors into a single comma-separated :not() list (review required for specificity drop).".to_string(),
                        selected: true,
                    });
                }
            }
            i += 1;
        }
    }

    plans
}

fn plan_merge_same_named_layers(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::MergeSameNamedLayer) {
        return;
    }
    let mut layer_groups: HashMap<String, Vec<&SourceNode>> = HashMap::new();
    for node in nodes {
        if let NodeKind::AtBlock { name, .. } = &node.kind
            && name == "layer"
        {
            let prelude = node.prelude(source).trim();
            if let Some(layer_name) = prelude.strip_prefix("@layer") {
                let layer_name = layer_name.trim();
                if !layer_name.is_empty() && !layer_name.contains('{') {
                    layer_groups
                        .entry(layer_name.to_string())
                        .or_default()
                        .push(node);
                }
            }
        }
    }

    let enabled_rules_vec: Vec<RuleId> = enabled.iter().copied().collect();

    for (layer_name, blocks) in layer_groups {
        if blocks.len() > 1 {
            let first = blocks[0];
            let parent_indent = line_indent(source, first.start);
            let first_body_range = first.body_range.as_ref().unwrap();
            let unit = detect_indent_unit(source, first_body_range.clone())
                .unwrap_or_else(|| "  ".to_string());
            let nested_indent = format!("{parent_indent}{unit}");

            let mut merged_body = String::new();
            for b in &blocks {
                if let Some(body_range) = &b.body_range {
                    let inner_nodes = scan_nodes(source, body_range.clone());
                    let inner_plans =
                        build_plans_recursive(path, source, &inner_nodes, &enabled_rules_vec);
                    let body_text = &source[body_range.clone()];
                    let modernized_body = if inner_plans.is_empty() {
                        body_text.to_string()
                    } else {
                        let mut local_plans = Vec::new();
                        for p in inner_plans {
                            if p.source_range.start >= body_range.start
                                && p.source_range.end <= body_range.end
                            {
                                let mut local_p = p.clone();
                                local_p.source_range.start -= body_range.start;
                                local_p.source_range.end -= body_range.start;
                                local_plans.push(local_p);
                            }
                        }
                        apply_selected_plans(body_text, &local_plans, true)
                            .unwrap_or_else(|_| body_text.to_string())
                    };

                    for line in modernized_body.lines() {
                        let trimmed = line.trim();
                        if !trimmed.is_empty() {
                            merged_body.push_str(&nested_indent);
                            merged_body.push_str(&ensure_semicolon(trimmed));
                            merged_body.push('\n');
                        }
                    }
                }
            }

            let proposed_first =
                format!("{parent_indent}@layer {layer_name} {{\n{merged_body}{parent_indent}}}");
            plans.push(PlanEntry {
                id: String::new(),
                file: path.to_path_buf(),
                rules: vec![RuleId::MergeSameNamedLayer],
                safety: Safety::Safe,
                source_range: SourceRange {
                    start: first.start,
                    end: first.end,
                },
                original: source[first.start..first.end].to_string(),
                proposed: proposed_first,
                proof: Proof::safe_local(),
                warnings: Vec::new(),
                reason: format!(
                    "Consolidate {} separated blocks of @layer {} into first occurrence.",
                    blocks.len(),
                    layer_name
                ),
                selected: true,
            });

            for subsequent in &blocks[1..] {
                plans.push(PlanEntry {
                    id: String::new(),
                    file: path.to_path_buf(),
                    rules: vec![RuleId::MergeSameNamedLayer],
                    safety: Safety::Safe,
                    source_range: SourceRange {
                        start: subsequent.start,
                        end: subsequent.end,
                    },
                    original: source[subsequent.start..subsequent.end].to_string(),
                    proposed: String::new(),
                    proof: Proof::safe_local(),
                    warnings: Vec::new(),
                    reason: format!(
                        "Remove consolidated subsequent block of @layer {}.",
                        layer_name
                    ),
                    selected: true,
                });
            }
        }
    }
}

fn plan_merge_adjacent_at_blocks(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if let NodeKind::AtBlock { name, .. } = &first.kind {
            let rule = match name.as_str() {
                "media" => RuleId::MergeAdjacentMedia,
                "supports" => RuleId::MergeAdjacentSupports,
                "container" => RuleId::MergeAdjacentContainer,
                "scope" => RuleId::MergeIdenticalScope,
                "starting-style" => RuleId::MergeIdenticalStartingStyle,
                _ => {
                    i += 1;
                    continue;
                }
            };

            if !enabled.contains(&rule) {
                i += 1;
                continue;
            }

            let first_prelude = first.prelude(source).trim();
            let mut cluster = vec![first];
            let mut cursor = i + 1;
            let mut prev_end = first.end;

            while cursor < nodes.len() {
                let next = &nodes[cursor];
                if !is_whitespace_only(source, prev_end..next.start) {
                    break;
                }
                if let NodeKind::AtBlock {
                    name: next_name, ..
                } = &next.kind
                    && next_name == name
                    && next.prelude(source).trim() == first_prelude
                {
                    cluster.push(next);
                    prev_end = next.end;
                    cursor += 1;
                    continue;
                }
                break;
            }

            if cluster.len() > 1 {
                let last = cluster.last().unwrap();
                let parent_indent = line_indent(source, first.start);
                let first_body_range = first.body_range.as_ref().unwrap();
                let unit = detect_indent_unit(source, first_body_range.clone())
                    .unwrap_or_else(|| "  ".to_string());
                let nested_indent = format!("{parent_indent}{unit}");

                let mut merged_body = String::new();
                for c in &cluster {
                    if let Some(body_range) = &c.body_range {
                        let body_text = &source[body_range.clone()];
                        for line in body_text.lines() {
                            let trimmed = line.trim();
                            if !trimmed.is_empty() {
                                merged_body.push_str(&nested_indent);
                                merged_body.push_str(&ensure_semicolon(trimmed));
                                merged_body.push('\n');
                            }
                        }
                    }
                }

                let proposed =
                    format!("{parent_indent}{first_prelude} {{\n{merged_body}{parent_indent}}}");
                plans.push(PlanEntry {
                    id: String::new(),
                    file: path.to_path_buf(),
                    rules: vec![rule],
                    safety: Safety::Safe,
                    source_range: SourceRange {
                        start: first.start,
                        end: last.end,
                    },
                    original: source[first.start..last.end].to_string(),
                    proposed,
                    proof: Proof::safe_local(),
                    warnings: Vec::new(),
                    reason: format!(
                        "Merge {} adjacent identical {} blocks into a single block.",
                        cluster.len(),
                        first_prelude
                    ),
                    selected: true,
                });
                i = cursor;
                continue;
            }
        }
        i += 1;
    }
}

fn plan_gather_consecutive_conditions_by_selector(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::NestMedia) && !enabled.contains(&RuleId::NestSupports) {
        return;
    }

    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if let NodeKind::AtBlock { name, .. } = &first.kind
            && (name == "media" || name == "supports")
            && let Some(target_sel) = extract_single_style_selector(source, first)
        {
            let mut cluster = vec![first];
            let mut cursor = i + 1;
            let mut prev_end = first.end;

            while cursor < nodes.len() {
                let next = &nodes[cursor];
                if !is_whitespace_only(source, prev_end..next.start) {
                    break;
                }
                if let NodeKind::AtBlock {
                    name: next_name, ..
                } = &next.kind
                    && (next_name == "media" || next_name == "supports")
                    && let Some(next_sel) = extract_single_style_selector(source, next)
                    && next_sel == target_sel
                {
                    cluster.push(next);
                    prev_end = next.end;
                    cursor += 1;
                    continue;
                }
                break;
            }

            if cluster.len() > 1 {
                let last = cluster.last().unwrap();
                let parent_indent = line_indent(source, first.start);
                let first_body_range = first.body_range.as_ref().unwrap();
                let unit = detect_indent_unit(source, first_body_range.clone())
                    .unwrap_or_else(|| "  ".to_string());
                let nested_indent = format!("{parent_indent}{unit}");
                let inner_decl_indent = format!("{nested_indent}{unit}");

                let mut body_out = String::new();
                for (idx, &c) in cluster.iter().enumerate() {
                    if idx > 0 {
                        body_out.push('\n');
                    }
                    let at_header = c.prelude(source).trim();
                    body_out.push_str(&nested_indent);
                    body_out.push_str(at_header);
                    body_out.push_str(" {\n");

                    let c_body_range = c.body_range.as_ref().unwrap();
                    let inner_nodes = scan_nodes(source, c_body_range.clone());
                    for in_node in &inner_nodes {
                        if let Some(in_body_range) = &in_node.body_range {
                            for line in source[in_body_range.clone()].lines() {
                                let trimmed = line.trim();
                                if !trimmed.is_empty() {
                                    body_out.push_str(&inner_decl_indent);
                                    body_out.push_str(&ensure_semicolon(trimmed));
                                    body_out.push('\n');
                                }
                            }
                        }
                    }

                    body_out.push_str(&nested_indent);
                    body_out.push_str("}\n");
                }

                let proposed =
                    format!("{parent_indent}{target_sel} {{\n{body_out}{parent_indent}}}");
                plans.push(PlanEntry {
                            id: String::new(),
                            file: path.to_path_buf(),
                            rules: vec![RuleId::NestMedia, RuleId::NestSupports],
                            safety: Safety::Safe,
                            source_range: SourceRange {
                                start: first.start,
                                end: last.end,
                            },
                            original: source[first.start..last.end].to_string(),
                            proposed,
                            proof: Proof::safe_local(),
                            warnings: Vec::new(),
                            reason: format!(
                                "Gather {} consecutive condition blocks targeting '{}' into a single component rule.",
                                cluster.len(),
                                target_sel
                            ),
                            selected: true,
                        });
                i = cursor;
                continue;
            }
        }
        i += 1;
    }
}

fn extract_single_style_selector<'a>(source: &'a str, at_node: &SourceNode) -> Option<&'a str> {
    let body_range = at_node.body_range.as_ref()?;
    let inner_nodes = scan_nodes(source, body_range.clone());
    if inner_nodes.len() == 1 && matches!(&inner_nodes[0].kind, NodeKind::Style) {
        Some(inner_nodes[0].prelude(source).trim())
    } else {
        None
    }
}

fn plan_nest_in_place_adjacent_states(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::NestPseudoClass) {
        return;
    }

    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if matches!(&first.kind, NodeKind::Style) {
            let first_sel = first.prelude(source).trim();
            if let Some(base) = extract_base_target(first_sel) {
                let mut cluster = vec![first];
                let mut cursor = i + 1;
                let mut prev_end = first.end;

                while cursor < nodes.len() {
                    let next = &nodes[cursor];
                    if !is_whitespace_only(source, prev_end..next.start) {
                        break;
                    }
                    if matches!(&next.kind, NodeKind::Style) {
                        let next_sel = next.prelude(source).trim();
                        if let Some(next_base) = extract_base_target(next_sel)
                            && next_base == base
                        {
                            cluster.push(next);
                            prev_end = next.end;
                            cursor += 1;
                            continue;
                        }
                    }
                    break;
                }

                if cluster.len() > 1 {
                    let last = cluster.last().unwrap();
                    let parent_indent = line_indent(source, first.start);
                    let first_body_range = first.body_range.as_ref().unwrap();
                    let unit = detect_indent_unit(source, first_body_range.clone())
                        .unwrap_or_else(|| "  ".to_string());
                    let nested_indent = format!("{parent_indent}{unit}");
                    let inner_decl_indent = format!("{nested_indent}{unit}");

                    let mut out = format!("{parent_indent}{base} {{\n");
                    for (idx, &c) in cluster.iter().enumerate() {
                        if idx > 0 {
                            out.push('\n');
                        }
                        let c_sel = c.prelude(source).trim();
                        let remainder = &c_sel[base.len()..];
                        let nested_sel = if remainder.starts_with(':')
                            || remainder.starts_with('[')
                            || remainder.starts_with('.')
                            || remainder.starts_with('#')
                        {
                            format!("&{remainder}")
                        } else {
                            remainder.trim().to_string()
                        };

                        out.push_str(&nested_indent);
                        out.push_str(&nested_sel);
                        out.push_str(" {\n");

                        if let Some(c_body_range) = &c.body_range {
                            for line in source[c_body_range.clone()].lines() {
                                let trimmed = line.trim();
                                if !trimmed.is_empty() {
                                    out.push_str(&inner_decl_indent);
                                    out.push_str(&ensure_semicolon(trimmed));
                                    out.push('\n');
                                }
                            }
                        }

                        out.push_str(&nested_indent);
                        out.push_str("}\n");
                    }

                    out.push_str(&parent_indent);
                    out.push('}');

                    plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::NestPseudoClass, RuleId::NestAttribute],
                        safety: Safety::Safe,
                        source_range: SourceRange {
                            start: first.start,
                            end: last.end,
                        },
                        original: source[first.start..last.end].to_string(),
                        proposed: out,
                        proof: Proof::safe_local(),
                        warnings: Vec::new(),
                        reason: format!(
                            "Nest {} adjacent state rules for '{}' in place without moving.",
                            cluster.len(),
                            base
                        ),
                        selected: true,
                    });
                    i = cursor;
                    continue;
                }
            }
        }
        i += 1;
    }
}

fn extract_base_target(selector: &str) -> Option<&str> {
    if contains_top_level_comma(selector) {
        return None;
    }
    if let Some(pos) = selector.find(':')
        && pos > 0
        && !selector[pos..].starts_with("::")
    {
        let base = selector[..pos].trim_end();
        if is_simple_nesting_base(base) {
            return Some(base);
        }
    }
    if let Some(pos) = selector.find('[')
        && pos > 0
    {
        let base = selector[..pos].trim_end();
        if is_simple_nesting_base(base) {
            return Some(base);
        }
    }
    None
}

fn is_simple_nesting_base(base: &str) -> bool {
    let first = base.chars().next();
    !base.is_empty()
        && !first.is_some_and(|c| matches!(c, '&' | ':' | '>' | '+' | '~'))
        && !base.chars().any(|c| c.is_whitespace())
        && !base.chars().any(|c| matches!(c, '>' | '+' | '~'))
}

fn parse_rule_body_items(body_str: &str) -> (Vec<String>, Vec<String>) {
    let mut declarations = Vec::new();
    let mut nested_rules = Vec::new();

    let mut depth = 0usize;
    let mut current_block = String::new();
    let mut current_decl = String::new();
    let mut in_comment = false;
    let bytes = body_str.as_bytes();
    let mut i = 0;

    while i < bytes.len() {
        if in_comment {
            current_decl.push(bytes[i] as char);
            current_block.push(bytes[i] as char);
            if bytes[i] == b'*' && i + 1 < bytes.len() && bytes[i + 1] == b'/' {
                current_decl.push('/');
                current_block.push('/');
                i += 2;
                in_comment = false;
                continue;
            }
            i += 1;
            continue;
        }

        if bytes[i] == b'/' && i + 1 < bytes.len() && bytes[i + 1] == b'*' {
            in_comment = true;
            current_decl.push('/');
            current_decl.push('*');
            current_block.push('/');
            current_block.push('*');
            i += 2;
            continue;
        }

        let b = bytes[i];
        if b == b'{' {
            depth += 1;
            if depth == 1 {
                current_block = current_decl.clone();
                current_decl.clear();
            }
            current_block.push('{');
            i += 1;
            continue;
        } else if b == b'}' {
            if depth > 0 {
                depth -= 1;
                current_block.push('}');
                if depth == 0 {
                    let trimmed = current_block.trim().to_string();
                    if !trimmed.is_empty() {
                        nested_rules.push(trimmed);
                    }
                    current_block.clear();
                    current_decl.clear();
                }
            }
            i += 1;
            continue;
        }

        if depth > 0 {
            current_block.push(b as char);
        } else {
            if b == b';' {
                current_decl.push(';');
                let trimmed = current_decl.trim().to_string();
                if !trimmed.is_empty() {
                    declarations.push(trimmed);
                }
                current_decl.clear();
            } else if b == b'\n' {
                let trimmed = current_decl.trim();
                // Selector-list continuations (`&:hover,` / `&::before,`) contain `:`
                // but are not declarations — they must stay attached to the `{` that follows.
                if !trimmed.is_empty()
                    && trimmed.contains(':')
                    && !trimmed.ends_with('{')
                    && !trimmed.ends_with(',')
                {
                    let rest = body_str[i + 1..].trim_start();
                    if !rest.starts_with('{') {
                        declarations.push(trimmed.to_string());
                        current_decl.clear();
                    } else {
                        current_decl.push('\n');
                    }
                } else {
                    current_decl.push('\n');
                }
            } else {
                current_decl.push(b as char);
            }
        }
        i += 1;
    }

    let trailing_decl = current_decl.trim().to_string();
    if !trailing_decl.is_empty() && trailing_decl.contains(':') {
        declarations.push(ensure_semicolon(&trailing_decl).into_owned());
    }

    (declarations, nested_rules)
}

fn is_ident_continue(c: char) -> bool {
    c.is_ascii_alphanumeric() || c == '-' || c == '_' || c == '\\'
}

/// MDN "Appending the `&` nesting selector": reverse the context so the
/// parent appears as a suffix (`&`) of the nested selector.
///
/// `.featured .card` → `.featured &`
/// `.featured.card`  → `.featured&`
/// `:not(.card)`     → `:not(&)`
/// `.foo > .card`    → `.foo > &`
fn appended_nesting_selector_raw(base: &str, candidate_sel: &str) -> Option<String> {
    let base = base.trim();
    let candidate_sel = candidate_sel.trim();
    if base.is_empty() || candidate_sel == base || contains_top_level_comma(candidate_sel) {
        return None;
    }

    const FNS: &[&str] = &[":not(", ":is(", ":where(", ":has("];
    for fn_name in FNS {
        let wrapped = format!("{fn_name}{base})");
        if candidate_sel == wrapped {
            return Some(format!("{fn_name}&)"));
        }
        if let Some(prefix) = candidate_sel.strip_suffix(wrapped.as_str())
            && !prefix.is_empty()
        {
            let prev = prefix.chars().last()?;
            if is_ident_continue(prev)
                || prev == '.'
                || prev == '#'
                || prev == ']'
                || prev == ')'
                || prev == '*'
            {
                return Some(format!("{prefix}{fn_name}&)"));
            }
        }
    }

    if !candidate_sel.ends_with(base) {
        return None;
    }
    let prefix = &candidate_sel[..candidate_sel.len() - base.len()];
    if prefix.is_empty() {
        return None;
    }
    let prev = prefix.chars().last()?;
    let first_of_base = base.chars().next()?;

    if prev.is_whitespace() || matches!(prev, '>' | '+' | '~') {
        return Some(format!("{prefix}&"));
    }

    // Compound join: `.featured.card`, `div.card` — the `.`/`#`/`[`/`:` of
    // `base` starts a new simple selector, not a mid-ident substring.
    if matches!(first_of_base, '.' | '#' | '[' | ':')
        && (is_ident_continue(prev) || prev == ']' || prev == ')' || prev == '*')
    {
        return Some(format!("{prefix}&"));
    }

    None
}

fn appended_nesting_selector(base: &str, candidate_sel: &str) -> Option<String> {
    let nested = appended_nesting_selector_raw(base, candidate_sel)?;
    // Refuse a nest that would be interpreted as a descendant. `&` anywhere
    // (including `:not(&)`) marks the selector as non-relative.
    selector_contains_nesting_amp(&nested).then_some(nested)
}

fn appended_selector_relation(parent: &str, child: &str) -> Option<(RelationKind, String)> {
    let nested = appended_nesting_selector(parent, child)?;
    let before_amp = nested.strip_suffix('&').unwrap_or(nested.as_str());
    let kind = if nested.contains(":not(")
        || nested.contains(":is(")
        || nested.contains(":where(")
        || nested.contains(":has(")
    {
        RelationKind::PseudoClass
    } else if before_amp.contains('>') || before_amp.contains('+') || before_amp.contains('~') {
        RelationKind::Combinator
    } else if nested.ends_with('&')
        && before_amp
            .chars()
            .last()
            .is_some_and(|c| !c.is_whitespace())
    {
        RelationKind::Compound
    } else {
        RelationKind::Descendant
    };
    Some((kind, nested))
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum RelatedKind {
    Prefix,
    Appended,
}

/// Render a combinator as one complete native CSS nested selector.
///
/// A combinator may be used directly in a nested selector (`> .child`), but
/// it must not become a rule on its own (`> { ... }`). Keeping `&` attached
/// here makes every construction path obey that invariant.
fn explicit_relative_combinator(combinator: char, selector: &str) -> String {
    format!("& {combinator} {}", selector.trim())
}

fn starts_with_explicit_combinator(selector: &str) -> bool {
    let Some(rest) = selector.trim().strip_prefix('&') else {
        return false;
    };
    rest.trim_start()
        .chars()
        .next()
        .is_some_and(|c| matches!(c, '>' | '+' | '~'))
}

fn prefix_related_nested_selector(base: &str, candidate_sel: &str) -> Option<String> {
    if candidate_sel == base || !candidate_sel.starts_with(base) {
        return None;
    }
    let rem_raw = &candidate_sel[base.len()..];
    let rem = rem_raw.trim_start();
    if rem.is_empty() {
        return None;
    }
    // Only attach '&' when the suffix is directly touching the base (no whitespace).
    // e.g. `.foo:hover` → `&:hover` but `.foo :not(*)` → `:not(*)` (descendant, no &).
    let directly_attached = !rem_raw.starts_with(|c: char| c.is_whitespace());
    if rem.starts_with(':') || rem.starts_with('[') || rem.starts_with('.') || rem.starts_with('#')
    {
        if directly_attached {
            return Some(format!("&{rem}"));
        } else {
            return Some(rem.to_string());
        }
    }
    if rem.starts_with('+') || rem.starts_with('>') || rem.starts_with('~') {
        let first_char = rem.chars().next()?;
        let rest = rem[1..].trim_start();
        return Some(explicit_relative_combinator(first_char, rest));
    }
    if rem_raw.starts_with(' ') {
        return Some(rem.to_string());
    }
    None
}

fn classify_related_nested_one(base: &str, candidate_sel: &str) -> Option<(RelatedKind, String)> {
    if candidate_sel == base || contains_top_level_comma(candidate_sel) {
        return None;
    }
    if let Some(rel) = prefix_related_nested_selector(base, candidate_sel) {
        return Some((RelatedKind::Prefix, rel));
    }
    appended_nesting_selector(base, candidate_sel).map(|rel| (RelatedKind::Appended, rel))
}

fn classify_related_comma_list(base: &str, candidate_sel: &str) -> Option<(RelatedKind, String)> {
    let parts: Vec<&str> = split_top_level_comma(candidate_sel)
        .into_iter()
        .map(str::trim)
        .filter(|s| !s.is_empty())
        .collect();
    if parts.len() < 2 {
        return None;
    }
    let mut rels = Vec::with_capacity(parts.len());
    let mut all_prefix = true;
    for part in parts {
        if part == base {
            rels.push("&".to_string());
            continue;
        }
        let (kind, rel) = classify_related_nested_one(base, part)?;
        if kind != RelatedKind::Prefix {
            all_prefix = false;
        }
        rels.push(rel);
    }
    let kind = if all_prefix {
        RelatedKind::Prefix
    } else {
        RelatedKind::Appended
    };
    Some((kind, rels.join(", ")))
}

fn classify_related_nested(base: &str, candidate_sel: &str) -> Option<(RelatedKind, String)> {
    if candidate_sel == base {
        return None;
    }
    if contains_top_level_comma(candidate_sel) {
        return classify_related_comma_list(base, candidate_sel);
    }
    classify_related_nested_one(base, candidate_sel)
}

fn extract_related_nested_selector(base: &str, candidate_sel: &str) -> Option<String> {
    classify_related_nested(base, candidate_sel).map(|(_, rel)| rel)
}

fn is_weak_gather_base(base: &str) -> bool {
    matches!(base.trim(), "*" | "html" | "body" | ":root" | ":host")
}

/// CSS Nesting: if a nested selector contains `&` anywhere (including inside
/// `:not()`, `:is()`, `:where()`, `:has()`), it is a *non-relative* selector.
/// The parent is not implicitly prepended as a descendant.
/// `.parent { .child:not(&) {} }` → `.child:not(:is(.parent))`, not
/// `.parent .child:not(.parent)`.
fn selector_contains_nesting_amp(selector: &str) -> bool {
    let bytes = selector.as_bytes();
    let mut i = 0;
    let mut quote: Option<u8> = None;
    let mut escaped = false;
    while i < bytes.len() {
        let b = bytes[i];
        if let Some(q) = quote {
            if escaped {
                escaped = false;
            } else if b == b'\\' {
                escaped = true;
            } else if b == q {
                quote = None;
            }
            i += 1;
            continue;
        }
        match b {
            b'\'' | b'"' => quote = Some(b),
            b'&' => return true,
            _ => {}
        }
        i += 1;
    }
    false
}

/// Last `/* … */` before `node_start` if only whitespace follows it.
fn leading_block_comment(source: &str, node_start: usize) -> Option<(usize, &str)> {
    let before = source.get(..node_start)?;
    let start = before.rfind("/*")?;
    let close_rel = source.get(start + 2..node_start)?.find("*/")?;
    let end = start + 2 + close_rel + 2;
    if !source[end..node_start]
        .bytes()
        .all(|b| b.is_ascii_whitespace())
    {
        return None;
    }
    Some((start, source[start..end].trim_end()))
}

fn is_simple_compound_selector(sel: &str) -> bool {
    let sel = sel.trim();
    if sel.is_empty()
        || sel.starts_with('&')
        || sel.starts_with('+')
        || sel.starts_with('>')
        || sel.starts_with('~')
        || contains_top_level_comma(sel)
    {
        return false;
    }
    let mut paren = 0usize;
    let mut brack = 0usize;
    for c in sel.chars() {
        match c {
            '(' => paren += 1,
            ')' => paren = paren.saturating_sub(1),
            '[' => brack += 1,
            ']' => brack = brack.saturating_sub(1),
            _ if paren == 0
                && brack == 0
                && (c.is_whitespace() || matches!(c, '+' | '>' | '~')) =>
            {
                return false;
            }
            _ => {}
        }
    }
    true
}

fn first_compound_stripped(sel: &str) -> Option<&str> {
    let sel = sel.trim();
    if sel.is_empty() || sel.starts_with('&') {
        return None;
    }
    let mut paren = 0usize;
    let mut brack = 0usize;
    let mut end = sel.len();
    for (i, c) in sel.char_indices() {
        match c {
            '(' => paren += 1,
            ')' => paren = paren.saturating_sub(1),
            '[' => brack += 1,
            ']' => brack = brack.saturating_sub(1),
            _ if paren == 0
                && brack == 0
                && (c.is_whitespace() || matches!(c, '+' | '>' | '~' | ',')) =>
            {
                end = i;
                break;
            }
            _ => {}
        }
    }
    let head = sel[..end].trim();
    if head.is_empty() || head.starts_with(':') || head.starts_with('[') {
        return None;
    }
    paren = 0;
    brack = 0;
    for (i, c) in head.char_indices() {
        match c {
            '(' => paren += 1,
            ')' => paren = paren.saturating_sub(1),
            '[' if paren == 0 && i > 0 => return Some(&head[..i]),
            ':' if paren == 0 && brack == 0 && i > 0 => return Some(&head[..i]),
            _ => {}
        }
    }
    Some(head)
}

fn style_body_weight(source: &str, node: &SourceNode) -> usize {
    node.body(source)
        .map(|b| b.lines().filter(|l| !l.trim().is_empty()).count())
        .unwrap_or(0)
}

/// Pick a single gather home for `sel`.
/// Exact existing rules win by specificity; prefix beats appended on a tie;
/// virtual stripped compounds (`.skip-link` from `.skip-link:hover`) are last resort.
fn assign_gather_home<'a>(
    sel: &str,
    exact_homes: &[&'a str],
    home_weight: &HashMap<&'a str, usize>,
) -> Option<&'a str> {
    #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
    struct Rank {
        spec: Specificity,
        is_prefix: bool,
        weight: usize,
        len: usize,
    }

    let mut best: Option<(Rank, &'a str)> = None;
    let consider =
        |best: &mut Option<(Rank, &'a str)>, home: &'a str, kind: Option<RelatedKind>| {
            let is_prefix = matches!(kind, None | Some(RelatedKind::Prefix));
            if matches!(kind, Some(RelatedKind::Appended)) && is_weak_gather_base(home) {
                return;
            }
            let rank = Rank {
                spec: calculate_specificity(home),
                is_prefix,
                weight: home_weight.get(home).copied().unwrap_or(0),
                len: home.len(),
            };
            if best.as_ref().is_none_or(|(cur, _)| rank > *cur) {
                *best = Some((rank, home));
            }
        };

    for &home in exact_homes {
        if sel == home || home_weight.get(home).copied().unwrap_or(0) == 0 {
            continue;
        }
        if let Some((kind, _)) = classify_related_nested(home, sel) {
            consider(&mut best, home, Some(kind));
        }
    }
    if best.is_some() {
        return best.map(|(_, home)| home);
    }
    // No existing rule claimed this selector — fall back to a virtual
    // stripped compound so `.skip-link:hover` + `.skip-link:focus` can
    // still wrap under a synthesized `.skip-link`.
    for &home in exact_homes {
        if sel == home || home_weight.get(home).copied().unwrap_or(0) != 0 {
            continue;
        }
        if let Some((kind, _)) = classify_related_nested(home, sel) {
            consider(&mut best, home, Some(kind));
        }
    }
    best.map(|(_, home)| home)
}

const GATHERABLE_CONDITIONALS: &[&str] = &["media", "supports", "container", "starting-style"];
/// `@layer` / `@scope` are NOT gather containers. Walking into them and
/// dumping members into the first home moves declarations across cascade
/// layers (e.g. `:root` in `@layer base` into `@layer tokens`).
const GATHER_SCAN_CONTAINERS: &[&str] = &[];

fn is_strong_gather_home(sel: &str) -> bool {
    let sel = sel.trim();
    sel.starts_with('.')
        || sel.starts_with('#')
        || sel.starts_with('[')
        || (sel.starts_with(':') && !sel.starts_with("::"))
}

fn is_absorbable_descendant(base: &str, sel: &str) -> bool {
    let sel = sel.trim();
    if sel.is_empty() || sel == base {
        return false;
    }
    if contains_top_level_comma(sel) {
        return split_top_level_comma(sel)
            .into_iter()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .all(|part| is_absorbable_descendant(base, part));
    }
    if extract_related_nested_selector(base, sel).is_some() {
        return true;
    }
    let head = first_compound_stripped(sel).unwrap_or(sel);
    if is_weak_gather_base(head) {
        return false;
    }
    if is_strong_gather_home(head) && head != first_compound_stripped(base).unwrap_or(base) {
        return false;
    }
    is_simple_compound_selector(sel) || first_compound_stripped(sel).is_some()
}

fn can_nest_under_gather_home(base: &str, sel: &str) -> bool {
    if selector_contains_nesting_amp(sel) {
        return false;
    }
    sel == base
        || extract_related_nested_selector(base, sel).is_some()
        || is_absorbable_descendant(base, sel)
}

enum GatherMember {
    Style(SourceNode),
    Conditional {
        at_node: SourceNode,
        inner: SourceNode,
        delete_whole_at_block: bool,
    },
}

impl GatherMember {
    fn outer_start(&self) -> usize {
        match self {
            Self::Style(n) => n.start,
            Self::Conditional { at_node, .. } => at_node.start,
        }
    }

    fn outer_end(&self) -> usize {
        match self {
            Self::Style(n) => n.end,
            Self::Conditional { at_node, .. } => at_node.end,
        }
    }
}

fn extend_with_trailing_newline(source: &str, end: usize) -> usize {
    if source[end..].starts_with("\r\n") {
        end + 2
    } else if source[end..].starts_with('\n') {
        end + 1
    } else {
        end
    }
}

fn push_relative_body_as_nest(rel_sel: &str, body_str: &str, all_nested_rules: &mut Vec<String>) {
    let (decls, nested) = parse_rule_body_items(body_str);
    if nested.is_empty() {
        let mut rel_body = String::new();
        for d in &decls {
            rel_body.push_str(&format!("{}\n", ensure_semicolon(d)));
        }
        all_nested_rules.push(format!("{rel_sel} {{\n    {rel_body}}}"));
    } else {
        let mut rel_body_lines = Vec::new();
        for d in &decls {
            rel_body_lines.push(format!("    {}", ensure_semicolon(d)));
        }
        for nr in &nested {
            rel_body_lines.push(nr.clone());
        }
        let rel_body = rel_body_lines.join("\n");
        all_nested_rules.push(format!("{rel_sel} {{\n{rel_body}\n}}"));
    }
}

fn push_style_member_into_merge(
    first_sel: &str,
    cand_sel: &str,
    body_str: &str,
    all_decls: &mut Vec<String>,
    all_nested_rules: &mut Vec<String>,
) {
    if cand_sel == first_sel {
        let (decls, nested) = parse_rule_body_items(body_str);
        all_decls.extend(decls);
        all_nested_rules.extend(nested);
    } else if let Some(rel_sel) = extract_related_nested_selector(first_sel, cand_sel) {
        push_relative_body_as_nest(&rel_sel, body_str, all_nested_rules);
    } else if is_absorbable_descendant(first_sel, cand_sel) {
        push_relative_body_as_nest(cand_sel, body_str, all_nested_rules);
    }
}

fn format_conditional_group_into_lines(
    first_sel: &str,
    at_header: &str,
    inners: &[&SourceNode],
    source: &str,
    nested_indent: &str,
    unit: &str,
) -> Vec<String> {
    let level2 = format!("{nested_indent}{unit}");
    let mut direct_decls = Vec::new();
    let mut nested = Vec::new();

    for inner in inners {
        let Some(body_range) = &inner.body_range else {
            continue;
        };
        let inner_sel = inner.prelude(source).trim();
        let inner_body = &source[body_range.clone()];
        if inner_sel == first_sel {
            let (decls, nrs) = parse_rule_body_items(inner_body);
            direct_decls.extend(decls);
            nested.extend(nrs);
        } else if let Some(rel_sel) = extract_related_nested_selector(first_sel, inner_sel) {
            push_relative_body_as_nest(&rel_sel, inner_body, &mut nested);
        }
    }

    let nested = factor_related_nested_rules(merge_same_prelude_nests(nested));
    let mut lines = Vec::new();
    lines.push(format!("{nested_indent}{at_header} {{"));
    for d in &direct_decls {
        lines.push(format!("{level2}{}", ensure_semicolon(d)));
    }
    for nr in &nested {
        if let Some(sel) = nested_rule_prelude(nr).map(str::to_string) {
            let item = NestedRuleItem {
                comment: nested_rule_comment_prefix(nr),
                sel,
                body: nested_rule_inner(nr),
            };
            for line in render_item_indented(&item, &level2, unit).lines() {
                lines.push(line.to_string());
            }
        } else {
            for line in nr.lines() {
                let trimmed = line.trim();
                if !trimmed.is_empty() {
                    lines.push(format!("{level2}{}", ensure_semicolon(trimmed)));
                }
            }
        }
    }
    lines.push(format!("{nested_indent}}}"));
    lines
}

fn format_conditional_into_lines(
    first_sel: &str,
    at_header: &str,
    inner_sel: &str,
    inner_body: &str,
    nested_indent: &str,
    unit: &str,
) -> Vec<String> {
    if inner_sel != first_sel {
        return Vec::new();
    }
    let (decls, nested) = parse_rule_body_items(inner_body);
    let level2 = format!("{nested_indent}{unit}");
    let mut lines = Vec::new();
    lines.push(format!("{nested_indent}{at_header} {{"));
    for d in &decls {
        lines.push(format!("{level2}{}", ensure_semicolon(d)));
    }
    for nr in &nested {
        if let Some(sel) = nested_rule_prelude(nr).map(str::to_string) {
            let item = NestedRuleItem {
                comment: nested_rule_comment_prefix(nr),
                sel,
                body: nested_rule_inner(nr),
            };
            for line in render_item_indented(&item, &level2, unit).lines() {
                lines.push(line.to_string());
            }
        } else {
            for line in nr.lines() {
                let trimmed = line.trim();
                if trimmed.is_empty() {
                    lines.push(String::new());
                } else {
                    lines.push(format!("{level2}{}", ensure_semicolon(trimmed)));
                }
            }
        }
    }
    lines.push(format!("{nested_indent}}}"));
    lines
}

fn nested_rule_prelude(nr: &str) -> Option<&str> {
    let mut i = 0usize;
    while i < nr.len() {
        while i < nr.len() && nr.as_bytes()[i].is_ascii_whitespace() {
            i += 1;
        }
        if nr[i..].starts_with("/*") {
            i += nr[i..].find("*/")? + 2;
            continue;
        }
        if nr[i..].starts_with("//") {
            i += nr[i..].find('\n').map(|n| n + 1).unwrap_or(nr.len() - i);
            continue;
        }
        break;
    }
    let rest = nr.get(i..)?;
    let brace = rest.find('{')?;
    let head = rest[..brace].trim();
    if head.is_empty() { None } else { Some(head) }
}

fn nested_rule_inner(nr: &str) -> String {
    let lines: Vec<&str> = nr.lines().collect();
    let open = lines.iter().position(|line| {
        let t = line.trim();
        t.ends_with('{') && !t.starts_with("/*") && !t.starts_with("//")
    });
    let Some(open) = open else {
        return String::new();
    };
    if lines.len() <= open + 2 {
        return String::new();
    }
    lines[open + 1..lines.len() - 1]
        .iter()
        .map(|l| l.trim_end())
        .collect::<Vec<_>>()
        .join("\n")
}

fn nested_rule_comment_prefix(nr: &str) -> String {
    let mut out = String::new();
    for line in nr.lines() {
        let t = line.trim();
        if t.is_empty() || t.starts_with("/*") || t.starts_with("//") {
            if !out.is_empty() {
                out.push('\n');
            }
            out.push_str(t);
        } else {
            break;
        }
    }
    out
}

/// Collapse `&.flash { a }` + `&.flash { @media { b } }` into one nest.
fn merge_same_prelude_nests(rules: Vec<String>) -> Vec<String> {
    let mut order: Vec<String> = Vec::new();
    let mut merged: HashMap<String, String> = HashMap::new();
    let mut comments: HashMap<String, String> = HashMap::new();
    let mut leftovers = Vec::new();

    for nr in rules {
        let Some(prelude) = nested_rule_prelude(&nr).map(str::to_string) else {
            leftovers.push(nr);
            continue;
        };
        let inner = nested_rule_inner(&nr);
        let prefix = nested_rule_comment_prefix(&nr);
        if let Some(existing) = merged.get_mut(&prelude) {
            if !existing.is_empty() && !inner.is_empty() {
                existing.push('\n');
            }
            existing.push_str(&inner);
        } else {
            order.push(prelude.clone());
            merged.insert(prelude.clone(), inner);
            if !prefix.is_empty() {
                comments.insert(prelude, prefix);
            }
        }
    }

    let mut out: Vec<String> = order
        .into_iter()
        .map(|prelude| {
            let inner = merged.remove(&prelude).unwrap_or_default();
            let comment = comments.remove(&prelude).unwrap_or_default();
            let head = if comment.is_empty() {
                String::new()
            } else {
                format!("{comment}\n")
            };
            if inner.is_empty() {
                format!("{head}{prelude} {{}}")
            } else {
                format!("{head}{prelude} {{\n{inner}\n}}")
            }
        })
        .collect();
    out.extend(leftovers);
    out
}

fn conditional_as_nested_rule(
    first_sel: &str,
    at_header: &str,
    inner_sel: &str,
    inner_body: &str,
) -> Option<String> {
    if inner_sel == first_sel {
        return None;
    }
    let rel_sel = extract_related_nested_selector(first_sel, inner_sel)?;
    let (decls, nested) = parse_rule_body_items(inner_body);
    let mut at_inner = String::new();
    for d in &decls {
        at_inner.push_str("        ");
        at_inner.push_str(&ensure_semicolon(d));
        at_inner.push('\n');
    }
    for nr in &nested {
        for line in nr.lines() {
            let trimmed = line.trim();
            if !trimmed.is_empty() {
                at_inner.push_str("        ");
                at_inner.push_str(&ensure_semicolon(trimmed));
                at_inner.push('\n');
            }
        }
    }
    Some(format!(
        "{rel_sel} {{\n    {at_header} {{\n{at_inner}    }}\n}}"
    ))
}

#[derive(Debug, Clone)]
struct NestedRuleItem {
    comment: String,
    sel: String,
    body: String,
}

fn render_nested_rule_item(item: &NestedRuleItem) -> String {
    let head = if item.comment.is_empty() {
        String::new()
    } else {
        format!("{}\n", item.comment)
    };
    if item.body.trim().is_empty() {
        format!("{head}{} {{}}", item.sel)
    } else {
        format!("{head}{} {{\n{}\n}}", item.sel, item.body)
    }
}

fn append_child_rule(parent: &mut NestedRuleItem, rel_sel: &str, child_body: &str) {
    let child = NestedRuleItem {
        comment: String::new(),
        sel: rel_sel.to_string(),
        body: child_body.to_string(),
    };
    if !parent.body.is_empty() {
        parent.body.push('\n');
    }
    parent.body.push_str(&render_nested_rule_item(&child));
}

fn split_relative_combinator(sel: &str) -> Option<(String, String)> {
    let mut paren = 0usize;
    let mut brack = 0usize;
    for (i, c) in sel.char_indices() {
        match c {
            '(' => paren += 1,
            ')' => paren = paren.saturating_sub(1),
            '[' => brack += 1,
            ']' => brack = brack.saturating_sub(1),
            _ if paren == 0 && brack == 0 => {
                if c.is_whitespace() {
                    let right = sel[i..].trim_start();
                    let left = sel[..i].trim();
                    if left.is_empty() || right.is_empty() {
                        return None;
                    }
                    return Some((left.to_string(), right.to_string()));
                }
                if matches!(c, '+' | '>' | '~') && i > 0 {
                    let right = sel[i + c.len_utf8()..].trim_start();
                    let left = sel[..i].trim();
                    if left.is_empty() || right.is_empty() {
                        return None;
                    }
                    return Some((left.to_string(), explicit_relative_combinator(c, right)));
                }
            }
            _ => {}
        }
    }
    None
}

fn is_at_rule_prelude(sel: &str) -> bool {
    sel.trim_start().starts_with('@')
}

fn synthesizable_parent(sel: &str) -> Option<(String, String)> {
    let sel = sel.trim();
    // A selector containing `&` is already expressed in native nesting
    // context. Treating its leading `&` as a synthetic parent would split a
    // valid `& > .child` into the invalid `& { > .child { ... } }` shape.
    if sel.is_empty()
        || sel == "&"
        || starts_with_explicit_combinator(sel)
        || is_at_rule_prelude(sel)
    {
        return None;
    }
    if contains_top_level_comma(sel) {
        let parts: Vec<&str> = split_top_level_comma(sel)
            .into_iter()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .collect();
        if parts.len() < 2 {
            return None;
        }
        let mut parent: Option<String> = None;
        let mut children = Vec::new();
        for part in parts {
            let (p, c) = synthesizable_parent(part)?;
            match &parent {
                Some(existing) if existing != &p => return None,
                None => parent = Some(p),
                _ => {}
            }
            children.push(c);
        }
        return Some((parent?, children.join(", ")));
    }
    split_relative_combinator(sel)
}

fn nest_items_under_existing(items: Vec<NestedRuleItem>) -> Vec<NestedRuleItem> {
    let n = items.len();
    if n < 2 {
        return items;
    }

    let mut parent_of: Vec<Option<usize>> = vec![None; n];
    for i in 0..n {
        let mut best: Option<(usize, usize)> = None;
        for j in 0..n {
            if i == j {
                continue;
            }
            // Only prefix relations. Appended `&` under an already-relative
            // sibling (`.select-arrow` ← `&:open .select-arrow`) would turn
            // `.custom-select:open .select-arrow` into `.select-arrow:open &`.
            if matches!(
                classify_related_nested(&items[j].sel, &items[i].sel),
                Some((RelatedKind::Prefix, _))
            ) {
                let len = items[j].sel.len();
                if best.is_none_or(|(_, l)| len >= l) {
                    best = Some((j, len));
                }
            }
        }
        if let Some((j, _)) = best {
            parent_of[i] = Some(j);
        }
    }

    // Only nest under roots so we do not have to rebuild intermediate parents.
    for i in 0..n {
        if let Some(j) = parent_of[i]
            && parent_of[j].is_some()
        {
            parent_of[i] = None;
        }
    }

    let mut out = Vec::new();
    let mut out_idx = vec![None; n];
    for i in 0..n {
        if parent_of[i].is_none() {
            out_idx[i] = Some(out.len());
            out.push(items[i].clone());
        }
    }
    for i in 0..n {
        if let Some(j) = parent_of[i]
            && let Some(out_j) = out_idx[j]
            && let Some((RelatedKind::Prefix, rel)) =
                classify_related_nested(&out[out_j].sel, &items[i].sel)
        {
            append_child_rule(&mut out[out_j], &rel, &items[i].body);
        }
    }
    out
}

fn wrap_shared_virtual_parents(items: Vec<NestedRuleItem>) -> Vec<NestedRuleItem> {
    #[derive(Clone)]
    enum Bucket {
        Atomic(NestedRuleItem),
        Shared {
            parent: String,
            originals: Vec<NestedRuleItem>,
            children: Vec<NestedRuleItem>,
        },
    }

    let mut buckets: Vec<Bucket> = Vec::new();
    let mut shared_at: HashMap<String, usize> = HashMap::new();

    for item in items {
        match synthesizable_parent(&item.sel) {
            Some((parent, child)) => {
                if let Some(&idx) = shared_at.get(&parent) {
                    if let Bucket::Shared {
                        originals,
                        children,
                        ..
                    } = &mut buckets[idx]
                    {
                        originals.push(item.clone());
                        children.push(NestedRuleItem {
                            comment: item.comment.clone(),
                            sel: child,
                            body: item.body,
                        });
                    }
                } else {
                    shared_at.insert(parent.clone(), buckets.len());
                    buckets.push(Bucket::Shared {
                        parent,
                        originals: vec![item.clone()],
                        children: vec![NestedRuleItem {
                            comment: item.comment.clone(),
                            sel: child,
                            body: item.body,
                        }],
                    });
                }
            }
            None => buckets.push(Bucket::Atomic(item)),
        }
    }

    let mut out = Vec::new();
    for bucket in buckets {
        match bucket {
            Bucket::Atomic(item) => out.push(item),
            Bucket::Shared {
                parent,
                originals,
                children,
            } => {
                if children.len() >= 2 {
                    let body = children
                        .iter()
                        .map(render_nested_rule_item)
                        .collect::<Vec<_>>()
                        .join("\n");
                    out.push(NestedRuleItem {
                        comment: String::new(),
                        sel: parent,
                        body,
                    });
                } else {
                    out.extend(originals);
                }
            }
        }
    }
    out
}

fn factor_related_nested_rules(rules: Vec<String>) -> Vec<String> {
    let mut items = Vec::new();
    let mut leftovers = Vec::new();
    for nr in rules {
        if let Some(sel) = nested_rule_prelude(&nr).map(str::to_string) {
            items.push(NestedRuleItem {
                comment: nested_rule_comment_prefix(&nr),
                sel,
                body: nested_rule_inner(&nr),
            });
        } else {
            leftovers.push(nr);
        }
    }

    for _ in 0..8 {
        let before = items.len();
        items = nest_items_under_existing(items);
        items = wrap_shared_virtual_parents(items);
        if items.len() == before {
            break;
        }
    }

    let mut out: Vec<String> = items.iter().map(render_nested_rule_item).collect();
    out.extend(leftovers);
    out
}

fn line_start(source: &str, pos: usize) -> usize {
    source[..pos.min(source.len())]
        .rfind('\n')
        .map(|i| i + 1)
        .unwrap_or(0)
}

fn relative_indent_unit(source: &str, node: &SourceNode) -> String {
    let parent = line_indent(source, node.start);
    let Some(body) = node.body_range.clone() else {
        return "    ".to_string();
    };
    match detect_indent_unit(source, body) {
        Some(raw) => raw
            .strip_prefix(parent.as_str())
            .filter(|rest| !rest.is_empty())
            .unwrap_or("    ")
            .to_string(),
        None => "    ".to_string(),
    }
}

fn squeeze_excess_blank_lines(s: &str) -> String {
    let ends_nl = s.ends_with('\n');
    let mut out = String::new();
    let mut blank_run = 0usize;
    for line in s.lines() {
        if line.trim().is_empty() {
            blank_run += 1;
            if blank_run > 1 {
                continue;
            }
            out.push('\n');
        } else {
            blank_run = 0;
            out.push_str(line);
            out.push('\n');
        }
    }
    if !ends_nl && out.ends_with('\n') {
        out.pop();
    }
    out
}

fn render_item_indented(item: &NestedRuleItem, indent: &str, unit: &str) -> String {
    let mut out = String::new();
    if !item.comment.is_empty() {
        for line in item.comment.lines() {
            out.push_str(indent);
            out.push_str(line.trim());
            out.push('\n');
        }
    }
    out.push_str(indent);
    out.push_str(&item.sel);
    out.push_str(" {\n");
    let inner = format!("{indent}{unit}");
    let (decls, nested) = parse_rule_body_items(&item.body);
    for d in decls {
        out.push_str(&inner);
        out.push_str(&ensure_semicolon(&d));
        out.push('\n');
    }
    for nr in nested {
        if let Some(sel) = nested_rule_prelude(&nr).map(str::to_string) {
            let child = NestedRuleItem {
                comment: nested_rule_comment_prefix(&nr),
                sel,
                body: nested_rule_inner(&nr),
            };
            out.push_str(&render_item_indented(&child, &inner, unit));
            out.push('\n');
        }
    }
    out.push_str(indent);
    out.push('}');
    out
}

fn leftover_style_replacements(
    source: &str,
    nodes: &[SourceNode],
    span_start: usize,
    span_end: usize,
    cluster_starts: &HashSet<usize>,
) -> Vec<(usize, usize, String)> {
    let leftovers: Vec<&SourceNode> = nodes
        .iter()
        .filter(|n| {
            matches!(n.kind, NodeKind::Style)
                && n.start >= span_start
                && n.end <= span_end
                && !cluster_starts.contains(&n.start)
        })
        .collect();

    let mut reps = Vec::new();
    let mut i = 0;
    while i < leftovers.len() {
        let parent = leftovers[i];
        let parent_sel = parent.prelude(source);
        if contains_top_level_comma(parent_sel) || parent_sel.contains("::") {
            i += 1;
            continue;
        }
        let mut children = Vec::new();
        let mut j = i + 1;
        let mut prev_end = parent.end;
        while j < leftovers.len() {
            let next = leftovers[j];
            if !is_whitespace_only(source, prev_end..next.start) {
                break;
            }
            if let Some((relation, nested_selector)) =
                selector_relation(parent_sel, next.prelude(source))
            {
                children.push(ClusterChild::Style {
                    node: (*next).clone(),
                    relation,
                    nested_selector,
                });
                prev_end = next.end;
                j += 1;
                continue;
            }
            break;
        }
        if !children.is_empty() {
            let last_end = children.last().expect("non-empty").node().end;
            reps.push((
                parent.start,
                last_end,
                render_cluster(source, parent, &children),
            ));
            i = j;
        } else {
            i += 1;
        }
    }
    reps
}

fn apply_range_replacements(
    source: &str,
    start: usize,
    end: usize,
    replacements: &[(usize, usize, String)],
) -> String {
    let mut reps: Vec<(usize, usize, &str)> = replacements
        .iter()
        .map(|(s, e, t)| (*s, *e, t.as_str()))
        .filter(|(s, e, _)| *s >= start && *e <= end && *s <= *e)
        .collect();
    reps.sort_by(|a, b| a.0.cmp(&b.0).then_with(|| b.1.cmp(&a.1)));
    let mut out = String::new();
    let mut cur = start;
    for (s, e, text) in reps {
        if s < cur {
            continue;
        }
        if s > cur {
            out.push_str(&source[cur..s]);
        }
        out.push_str(text);
        cur = e;
    }
    if cur < end {
        out.push_str(&source[cur..end]);
    }
    out
}

fn is_gather_delete(plan: &PlanEntry) -> bool {
    plan.proposed.is_empty() && plan.rules.contains(&RuleId::GatherRelatedSelectorRules)
}

fn gather_target_key(plan: &PlanEntry) -> Option<String> {
    if !plan.rules.contains(&RuleId::GatherRelatedSelectorRules) {
        return None;
    }
    let start = plan.reason.find("for '")? + 5;
    let rest = plan.reason.get(start..)?;
    let end = rest.find('\'')?;
    Some(rest[..end].to_string())
}

fn ranges_overlap(a: &SourceRange, b: &SourceRange) -> bool {
    a.start < b.end && b.start < a.end
}

fn select_disjoint_plan_indices(plans: &[PlanEntry]) -> Vec<usize> {
    let mut primary: Vec<usize> = (0..plans.len())
        .filter(|&i| !is_gather_delete(&plans[i]))
        .collect();
    primary.sort_by(|&i, &j| {
        plans[i]
            .source_range
            .start
            .cmp(&plans[j].source_range.start)
            .then_with(|| plans[j].source_range.end.cmp(&plans[i].source_range.end))
    });

    let mut kept = Vec::new();
    let mut last_end = 0usize;
    for i in primary {
        if plans[i].source_range.start >= last_end {
            last_end = plans[i].source_range.end;
            kept.push(i);
        }
    }

    let mut kept_gather_keys = HashSet::new();
    for &i in &kept {
        if !plans[i].proposed.is_empty()
            && let Some(key) = gather_target_key(&plans[i])
        {
            kept_gather_keys.insert((plans[i].file.clone(), key));
        }
    }

    let mut deletes: Vec<usize> = (0..plans.len())
        .filter(|&i| is_gather_delete(&plans[i]))
        .collect();
    deletes.sort_by(|&i, &j| {
        plans[i]
            .source_range
            .start
            .cmp(&plans[j].source_range.start)
    });

    for i in deletes {
        let Some(key) = gather_target_key(&plans[i]) else {
            continue;
        };
        if !kept_gather_keys.contains(&(plans[i].file.clone(), key)) {
            continue;
        }
        if kept
            .iter()
            .any(|&k| ranges_overlap(&plans[k].source_range, &plans[i].source_range))
        {
            continue;
        }
        kept.push(i);
    }

    kept.sort_by(|&i, &j| {
        plans[i]
            .source_range
            .start
            .cmp(&plans[j].source_range.start)
            .then_with(|| plans[j].source_range.end.cmp(&plans[i].source_range.end))
    });
    kept
}

fn format_merged_rule(
    first_sel: &str,
    parent_indent: &str,
    unit: &str,
    cluster: &[GatherMember],
    source: &str,
) -> String {
    let nested_indent = format!("{parent_indent}{unit}");

    let mut all_decls = Vec::new();
    let mut all_nested_rules = Vec::new();
    let mut absorbed_nests = Vec::new();
    let mut conditional_lines = Vec::new();

    for member in cluster {
        let GatherMember::Style(c) = member else {
            continue;
        };
        if let Some(body_range) = &c.body_range {
            let cand_sel = c.prelude(source).trim();
            let body_str = &source[body_range.clone()];
            let before_len = all_nested_rules.len();
            push_style_member_into_merge(
                first_sel,
                cand_sel,
                body_str,
                &mut all_decls,
                &mut all_nested_rules,
            );
            if all_nested_rules.len() > before_len
                && let Some((_, cmt)) = leading_block_comment(source, c.start)
                && let Some(last) = all_nested_rules.last_mut()
            {
                *last = format!("{cmt}\n{last}");
            }
        }
    }

    let mut cond_order: Vec<usize> = Vec::new();
    let mut cond_groups: HashMap<usize, (&SourceNode, Vec<&SourceNode>)> = HashMap::new();
    for member in cluster {
        let GatherMember::Conditional { at_node, inner, .. } = member else {
            continue;
        };
        cond_groups
            .entry(at_node.start)
            .and_modify(|(_, inners)| inners.push(inner))
            .or_insert_with(|| {
                cond_order.push(at_node.start);
                (at_node, vec![inner])
            });
    }

    for key in cond_order {
        let Some((at_node, inners)) = cond_groups.remove(&key) else {
            continue;
        };
        let at_header = at_node.prelude(source).trim();
        let mut related = Vec::new();
        let mut absorbed = Vec::new();
        for inner in inners {
            let sel = inner.prelude(source).trim();
            if sel == first_sel || extract_related_nested_selector(first_sel, sel).is_some() {
                related.push(inner);
            } else {
                absorbed.push(inner);
            }
        }

        if !related.is_empty() {
            let invert_group = related.len() >= 2
                || related
                    .iter()
                    .any(|inner| inner.prelude(source).trim() == first_sel);
            if invert_group {
                let extra = format_conditional_group_into_lines(
                    first_sel,
                    at_header,
                    &related,
                    source,
                    &nested_indent,
                    unit,
                );
                if !conditional_lines.is_empty() && !extra.is_empty() {
                    conditional_lines.push(String::new());
                }
                conditional_lines.extend(extra);
            } else {
                for inner in &related {
                    if let Some(body_range) = &inner.body_range {
                        let inner_sel = inner.prelude(source).trim();
                        let inner_body = &source[body_range.clone()];
                        if let Some(nr) =
                            conditional_as_nested_rule(first_sel, at_header, inner_sel, inner_body)
                        {
                            all_nested_rules.push(nr);
                        } else {
                            let extra = format_conditional_into_lines(
                                first_sel,
                                at_header,
                                inner_sel,
                                inner_body,
                                &nested_indent,
                                unit,
                            );
                            if !conditional_lines.is_empty() && !extra.is_empty() {
                                conditional_lines.push(String::new());
                            }
                            conditional_lines.extend(extra);
                        }
                    }
                }
            }
        }

        for inner in absorbed {
            if let Some(body_range) = &inner.body_range {
                push_style_member_into_merge(
                    first_sel,
                    inner.prelude(source).trim(),
                    &source[body_range.clone()],
                    &mut all_decls,
                    &mut absorbed_nests,
                );
            }
        }
    }

    let all_nested_rules = factor_related_nested_rules(merge_same_prelude_nests(all_nested_rules));
    let absorbed_nests = factor_related_nested_rules(merge_same_prelude_nests(absorbed_nests));

    let mut body_lines = Vec::new();

    for d in &all_decls {
        body_lines.push(format!("{nested_indent}{}", ensure_semicolon(d)));
    }

    if !all_decls.is_empty() && !all_nested_rules.is_empty() {
        body_lines.push(String::new());
    }

    for (idx, nr) in all_nested_rules.iter().enumerate() {
        if let Some(sel) = nested_rule_prelude(nr).map(str::to_string) {
            let item = NestedRuleItem {
                comment: nested_rule_comment_prefix(nr),
                sel,
                body: nested_rule_inner(nr),
            };
            body_lines.push(render_item_indented(&item, &nested_indent, unit));
        } else {
            for line in nr.lines() {
                let trimmed = line.trim();
                if trimmed.is_empty() {
                    body_lines.push(String::new());
                } else {
                    body_lines.push(format!("{nested_indent}{}", ensure_semicolon(trimmed)));
                }
            }
        }

        if idx < all_nested_rules.len() - 1 {
            body_lines.push(String::new());
        }
    }

    if !conditional_lines.is_empty() {
        if !body_lines.is_empty() {
            body_lines.push(String::new());
        }
        body_lines.extend(conditional_lines);
    }

    if !absorbed_nests.is_empty() {
        if !body_lines.is_empty() {
            body_lines.push(String::new());
        }
        for (idx, nr) in absorbed_nests.iter().enumerate() {
            if let Some(sel) = nested_rule_prelude(nr).map(str::to_string) {
                let item = NestedRuleItem {
                    comment: nested_rule_comment_prefix(nr),
                    sel,
                    body: nested_rule_inner(nr),
                };
                body_lines.push(render_item_indented(&item, &nested_indent, unit));
            } else {
                for line in nr.lines() {
                    let trimmed = line.trim();
                    if trimmed.is_empty() {
                        body_lines.push(String::new());
                    } else {
                        body_lines.push(format!("{nested_indent}{}", ensure_semicolon(trimmed)));
                    }
                }
            }
            if idx < absorbed_nests.len() - 1 {
                body_lines.push(String::new());
            }
        }
    }

    let body_content = body_lines.join("\n");
    format!("{parent_indent}{first_sel} {{\n{body_content}\n{parent_indent}}}")
}

fn plan_merge_adjacent_identical_selectors(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::MergeAdjacentIdenticalSelector) {
        return;
    }
    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if matches!(&first.kind, NodeKind::Style) {
            let first_sel = first.prelude(source).trim();
            let mut cluster = vec![first];
            let mut cursor = i + 1;
            let mut prev_end = first.end;

            while cursor < nodes.len() {
                let next = &nodes[cursor];
                if !is_whitespace_only(source, prev_end..next.start) {
                    break;
                }
                if matches!(&next.kind, NodeKind::Style) && next.prelude(source).trim() == first_sel
                {
                    cluster.push(next);
                    prev_end = next.end;
                    cursor += 1;
                    continue;
                }
                break;
            }

            if cluster.len() > 1 {
                let last = cluster.last().unwrap();
                let parent_indent = line_indent(source, first.start);
                let first_body_range = first.body_range.as_ref().unwrap();
                let unit = detect_indent_unit(source, first_body_range.clone())
                    .unwrap_or_else(|| "    ".to_string());

                let members: Vec<GatherMember> = cluster
                    .iter()
                    .map(|n| GatherMember::Style((*n).clone()))
                    .collect();
                let proposed =
                    format_merged_rule(first_sel, &parent_indent, &unit, &members, source);

                plans.push(PlanEntry {
                    id: String::new(),
                    file: path.to_path_buf(),
                    rules: vec![RuleId::MergeAdjacentIdenticalSelector],
                    safety: Safety::Safe,
                    source_range: SourceRange {
                        start: first.start,
                        end: last.end,
                    },
                    original: source[first.start..last.end].to_string(),
                    proposed,
                    proof: Proof::safe_local(),
                    warnings: Vec::new(),
                    reason: format!(
                        "Merge {} adjacent identical selector rules for '{}' into a single block.",
                        cluster.len(),
                        first_sel
                    ),
                    selected: true,
                });
                i = cursor;
                continue;
            }
        }
        i += 1;
    }
}

fn note_gather_home<'s>(
    source: &'s str,
    node: &SourceNode,
    exact_homes: &mut Vec<&'s str>,
    home_weight: &mut HashMap<&'s str, usize>,
) {
    let sel = node.prelude(source).trim();
    let core = first_compound_stripped(sel).unwrap_or(sel);
    if is_simple_compound_selector(sel) && core == sel && !sel.contains("::") {
        if !exact_homes.contains(&sel) {
            exact_homes.push(sel);
        }
        *home_weight.entry(sel).or_insert(0) += style_body_weight(source, node);
    }
    if let Some(stripped) = first_compound_stripped(sel)
        && !exact_homes.contains(&stripped)
        && is_simple_compound_selector(stripped)
        && !stripped.contains("::")
    {
        exact_homes.push(stripped);
        home_weight.entry(stripped).or_insert(0);
    }
}

fn collect_gather_homes<'s>(
    source: &'s str,
    nodes: &[SourceNode],
    exact_homes: &mut Vec<&'s str>,
    home_weight: &mut HashMap<&'s str, usize>,
) {
    for node in nodes {
        if matches!(&node.kind, NodeKind::Style) {
            note_gather_home(source, node, exact_homes, home_weight);
        } else if let NodeKind::AtBlock { name, .. } = &node.kind
            && GATHER_SCAN_CONTAINERS.contains(&name.as_str())
            && let Some(body_range) = &node.body_range
        {
            let inner = scan_nodes(source, body_range.clone());
            collect_gather_homes(source, &inner, exact_homes, home_weight);
        }
    }
}

fn should_group_at_block(
    name: &str,
    source: &str,
    style_inners: &[&SourceNode],
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
) -> bool {
    if style_inners.len() < 3 {
        return false;
    }
    // `@starting-style` has temporal semantics in addition to selector
    // semantics. Do not gather a nested selector into a shorter selector
    // home, because that can change the subject receiving the transition
    // starting state.
    if name == "starting-style"
        && style_inners.iter().any(|inner| {
            let sel = inner.prelude(source).trim();
            assign_gather_home(sel, exact_homes, home_weight).is_some_and(|home| home != sel)
        })
    {
        return false;
    }
    let mut assigned: HashMap<&str, usize> = HashMap::new();
    let mut unsafe_extract = false;
    for inner in style_inners {
        let sel = inner.prelude(source).trim();
        match assign_gather_home(sel, exact_homes, home_weight) {
            Some(home) => {
                *assigned.entry(home).or_insert(0) += 1;
                if home != sel
                    && extract_related_nested_selector(home, sel).is_none()
                    && !is_absorbable_descendant(home, sel)
                {
                    unsafe_extract = true;
                }
            }
            None => {
                if contains_top_level_comma(sel)
                    && exact_homes
                        .iter()
                        .any(|home| extract_related_nested_selector(home, sel).is_some())
                {
                    // rewriteable comma list — do not force-group
                } else {
                    *assigned.entry("").or_insert(0) += 1;
                    unsafe_extract = true;
                }
            }
        }
    }
    let strong: Vec<(&str, usize)> = assigned
        .iter()
        .filter(|(home, _)| !home.is_empty() && is_strong_gather_home(home))
        .map(|(h, c)| (*h, *c))
        .collect();
    let total = style_inners.len();
    let dominant = strong.iter().any(|(_, c)| *c * 3 >= total * 2);

    if name == "media" || name == "container" {
        return unsafe_extract || strong.len() >= 2;
    }
    // @supports / @starting-style: invert when one component owns the block.
    if strong.len() >= 2 && !dominant {
        return true;
    }
    unsafe_extract && strong.is_empty()
}

fn mark_grouped_at_blocks(
    source: &str,
    nodes: &[SourceNode],
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
    grouped: &mut HashSet<usize>,
) {
    for node in nodes {
        if let NodeKind::AtBlock { name, .. } = &node.kind
            && let Some(body_range) = &node.body_range
        {
            let inner_nodes = scan_nodes(source, body_range.clone());
            if GATHERABLE_CONDITIONALS.contains(&name.as_str()) {
                let style_inners: Vec<&SourceNode> = inner_nodes
                    .iter()
                    .filter(|n| matches!(&n.kind, NodeKind::Style))
                    .collect();
                if should_group_at_block(name, source, &style_inners, exact_homes, home_weight) {
                    grouped.insert(node.start);
                }
            }
            if GATHERABLE_CONDITIONALS.contains(&name.as_str())
                || GATHER_SCAN_CONTAINERS.contains(&name.as_str())
            {
                mark_grouped_at_blocks(source, &inner_nodes, exact_homes, home_weight, grouped);
            }
        }
    }
}

fn style_belongs_to_home(
    source: &str,
    node: &SourceNode,
    base: &str,
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
) -> bool {
    let sel = node.prelude(source).trim();
    sel == base || assign_gather_home(sel, exact_homes, home_weight) == Some(base)
}

fn collect_gather_cluster(
    source: &str,
    nodes: &[SourceNode],
    base: &str,
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
    grouped_at_blocks: &HashSet<usize>,
    cluster: &mut Vec<GatherMember>,
) {
    for node in nodes {
        if matches!(&node.kind, NodeKind::Style) {
            if style_belongs_to_home(source, node, base, exact_homes, home_weight) {
                cluster.push(GatherMember::Style(node.clone()));
            }
        } else if let NodeKind::AtBlock { name, .. } = &node.kind {
            let Some(body_range) = &node.body_range else {
                continue;
            };
            let inner_nodes = scan_nodes(source, body_range.clone());
            if GATHER_SCAN_CONTAINERS.contains(&name.as_str()) {
                collect_gather_cluster(
                    source,
                    &inner_nodes,
                    base,
                    exact_homes,
                    home_weight,
                    grouped_at_blocks,
                    cluster,
                );
                continue;
            }
            if grouped_at_blocks.contains(&node.start) {
                continue;
            }
            if !GATHERABLE_CONDITIONALS.contains(&name.as_str()) {
                continue;
            }
            // A conditional block containing another at-rule is a structured
            // subtree. Extracting only its direct style children would make
            // the replacement span delete nested conditionals (for example
            // `@supports { details { ... } @starting-style { ... } }`).
            // Leave mixed conditional contents grouped for a later, dedicated
            // rule rather than risking data loss.
            if inner_nodes
                .iter()
                .any(|inner| !matches!(&inner.kind, NodeKind::Style))
            {
                continue;
            }
            let style_inners: Vec<&SourceNode> = inner_nodes
                .iter()
                .filter(|n| matches!(n.kind, NodeKind::Style))
                .collect();
            let owned = style_inners
                .iter()
                .filter(|inner| {
                    let sel = inner.prelude(source).trim();
                    (name != "starting-style"
                        && style_belongs_to_home(source, inner, base, exact_homes, home_weight))
                        || (name == "starting-style" && sel == base)
                })
                .count();
            let dominate = !style_inners.is_empty() && owned * 3 >= style_inners.len() * 2;
            let related: Vec<SourceNode> = inner_nodes
                .iter()
                .filter(|inner| {
                    matches!(&inner.kind, NodeKind::Style) && {
                        let sel = inner.prelude(source).trim();
                        ((name != "starting-style"
                            && style_belongs_to_home(
                                source,
                                inner,
                                base,
                                exact_homes,
                                home_weight,
                            ))
                            || (name == "starting-style" && sel == base))
                            || (name != "starting-style"
                                && dominate
                                && is_absorbable_descendant(base, sel))
                    }
                })
                .cloned()
                .collect();
            if !related.is_empty() {
                let delete_whole_at_block = related.len() == inner_nodes.len()
                    || (dominate
                        && style_inners.iter().all(|inner| {
                            let sel = inner.prelude(source).trim();
                            ((name != "starting-style"
                                && style_belongs_to_home(
                                    source,
                                    inner,
                                    base,
                                    exact_homes,
                                    home_weight,
                                ))
                                || (name == "starting-style" && sel == base))
                                || (name != "starting-style" && is_absorbable_descendant(base, sel))
                        }));
                for inner in related {
                    cluster.push(GatherMember::Conditional {
                        at_node: node.clone(),
                        inner,
                        delete_whole_at_block,
                    });
                }
            }
        }
    }
}

fn enclosing_container_end(
    source: &str,
    nodes: &[SourceNode],
    first: &SourceNode,
) -> Option<usize> {
    fn find(source: &str, nodes: &[SourceNode], first: &SourceNode) -> Option<usize> {
        for node in nodes {
            if let NodeKind::AtBlock { name, .. } = &node.kind
                && let Some(body_range) = &node.body_range
                && (GATHER_SCAN_CONTAINERS.contains(&name.as_str())
                    || GATHERABLE_CONDITIONALS.contains(&name.as_str()))
                && first.start >= body_range.start
                && first.end <= body_range.end
            {
                let inner = scan_nodes(source, body_range.clone());
                if let Some(deeper) = find(source, &inner, first) {
                    return Some(deeper);
                }
                return Some(body_range.end);
            }
        }
        None
    }
    find(source, nodes, first)
}

fn named_layer_ident(prelude: &str) -> Option<String> {
    let rest = prelude
        .trim()
        .strip_prefix("@layer")
        .unwrap_or(prelude)
        .trim();
    if rest.is_empty() || contains_top_level_comma(rest) {
        return None;
    }
    let ident: String = rest
        .chars()
        .take_while(|c| c.is_ascii_alphanumeric() || matches!(c, '-' | '_' | '.'))
        .collect();
    if ident.is_empty() { None } else { Some(ident) }
}

#[derive(Clone)]
struct LayeredExactHit {
    selector: String,
    layer_path: Vec<String>,
    style: SourceNode,
    layer: SourceNode,
}

fn collect_layered_exact_hits(
    source: &str,
    nodes: &[SourceNode],
    path: &[String],
    current_layer: Option<&SourceNode>,
    out: &mut Vec<LayeredExactHit>,
) {
    for node in nodes {
        if matches!(node.kind, NodeKind::Style) {
            if let Some(layer) = current_layer
                && !path.is_empty()
            {
                let sel = node.prelude(source).trim();
                if !sel.is_empty() && !contains_top_level_comma(sel) {
                    out.push(LayeredExactHit {
                        selector: sel.to_string(),
                        layer_path: path.to_vec(),
                        style: node.clone(),
                        layer: layer.clone(),
                    });
                }
            }
            continue;
        }
        if let NodeKind::AtBlock { name, .. } = &node.kind
            && name == "layer"
            && let Some(body_range) = &node.body_range
            && let Some(ident) = named_layer_ident(node.prelude(source))
        {
            let mut child = path.to_vec();
            child.push(ident);
            let inner = scan_nodes(source, body_range.clone());
            collect_layered_exact_hits(source, &inner, &child, Some(node), out);
        }
    }
}

fn unwrap_style_block_body(block: &str) -> String {
    let open = match block.find('{') {
        Some(i) => i + 1,
        None => return block.to_string(),
    };
    let close = match block.rfind('}') {
        Some(i) => i,
        None => return block[open..].to_string(),
    };
    if close <= open {
        return String::new();
    }
    block[open..close].trim().to_string()
}

fn reindent_owned(text: &str, indent: &str) -> Vec<String> {
    let lines: Vec<&str> = text.lines().collect();
    let min_pad = lines
        .iter()
        .filter_map(|l| {
            if l.trim().is_empty() {
                None
            } else {
                Some(l.len() - l.trim_start().len())
            }
        })
        .min()
        .unwrap_or(0);
    lines
        .into_iter()
        .map(|l| {
            if l.trim().is_empty() {
                String::new()
            } else {
                let rest = if l.len() >= min_pad {
                    &l[min_pad..]
                } else {
                    l.trim_start()
                };
                format!("{indent}{rest}")
            }
        })
        .collect()
}

fn layer_body_cluster_for_selector(
    source: &str,
    layer: &SourceNode,
    selector: &str,
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
) -> Vec<GatherMember> {
    let Some(body_range) = &layer.body_range else {
        return Vec::new();
    };
    let inner = scan_nodes(source, body_range.clone());
    let mut cluster = Vec::new();
    for node in &inner {
        if matches!(node.kind, NodeKind::Style) && node.prelude(source).trim() == selector {
            cluster.push(GatherMember::Style(node.clone()));
        } else if let NodeKind::AtBlock { name, .. } = &node.kind
            && GATHERABLE_CONDITIONALS.contains(&name.as_str())
            && let Some(at_body) = &node.body_range
        {
            let at_inners = scan_nodes(source, at_body.clone());
            let related: Vec<SourceNode> = at_inners
                .iter()
                .filter(|n| {
                    matches!(n.kind, NodeKind::Style)
                        && (n.prelude(source).trim() == selector
                            || style_belongs_to_home(source, n, selector, exact_homes, home_weight))
                })
                .cloned()
                .collect();
            if related.is_empty() {
                continue;
            }
            let delete_whole = related.len() == at_inners.len();
            for inner_style in related {
                cluster.push(GatherMember::Conditional {
                    at_node: node.clone(),
                    inner: inner_style,
                    delete_whole_at_block: delete_whole,
                });
            }
        }
    }
    cluster
}

fn layer_contains_only_selector_cluster(
    source: &str,
    layer: &SourceNode,
    selector: &str,
    exact_homes: &[&str],
    home_weight: &HashMap<&str, usize>,
) -> bool {
    let Some(body_range) = &layer.body_range else {
        return false;
    };
    let inner = scan_nodes(source, body_range.clone());
    if inner.is_empty() {
        return false;
    }
    inner.iter().all(|node| {
        if matches!(node.kind, NodeKind::Style) {
            return node.prelude(source).trim() == selector
                || style_belongs_to_home(source, node, selector, exact_homes, home_weight);
        }
        if let NodeKind::AtBlock { name, .. } = &node.kind
            && GATHERABLE_CONDITIONALS.contains(&name.as_str())
            && let Some(at_body) = &node.body_range
        {
            let at_inners = scan_nodes(source, at_body.clone());
            return !at_inners.is_empty()
                && at_inners.iter().all(|n| {
                    matches!(n.kind, NodeKind::Style)
                        && (n.prelude(source).trim() == selector
                            || style_belongs_to_home(source, n, selector, exact_homes, home_weight))
                });
        }
        false
    })
}

fn intervening_unlayered(
    nodes: &[SourceNode],
    first_layer_start: usize,
    last_layer_end: usize,
) -> bool {
    nodes.iter().any(|n| {
        n.start > first_layer_start
            && n.end < last_layer_end
            && match &n.kind {
                NodeKind::Style => true,
                NodeKind::AtBlock { name, .. } => name != "layer",
                NodeKind::AtStatement { name, .. } => name != "layer",
            }
    })
}

fn plan_nest_layer_by_selector(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::NestLayerBySelector) {
        return;
    }

    let mut hits = Vec::new();
    collect_layered_exact_hits(source, nodes, &[], None, &mut hits);
    if hits.is_empty() {
        return;
    }

    let mut exact_homes: Vec<&str> = Vec::new();
    let mut home_weight: HashMap<&str, usize> = HashMap::new();
    collect_gather_homes(source, nodes, &mut exact_homes, &mut home_weight);

    let mut by_sel: HashMap<String, Vec<LayeredExactHit>> = HashMap::new();
    for hit in hits {
        by_sel.entry(hit.selector.clone()).or_default().push(hit);
    }

    for (selector, mut group) in by_sel {
        group.sort_by_key(|h| h.style.start);
        let mut seen_paths: Vec<Vec<String>> = Vec::new();
        for hit in &group {
            if !seen_paths.iter().any(|p| p == &hit.layer_path) {
                seen_paths.push(hit.layer_path.clone());
            }
        }
        if seen_paths.len() < 2 {
            continue;
        }

        let first_layer = &group[0].layer;
        let last_layer = &group.last().unwrap().layer;
        if intervening_unlayered(nodes, first_layer.start, last_layer.end) {
            continue;
        }

        let parent_indent = String::new();
        let unit = "    ".to_string();
        let mut layer_blocks = Vec::new();
        let mut deletes: Vec<(usize, usize, usize)> = Vec::new();
        let mut deleted_layers = HashSet::new();

        for path in &seen_paths {
            let Some(hit) = group.iter().find(|h| &h.layer_path == path) else {
                continue;
            };
            let cluster = layer_body_cluster_for_selector(
                source,
                &hit.layer,
                &selector,
                &exact_homes,
                &home_weight,
            );
            if cluster.is_empty() {
                continue;
            }
            let merged = format_merged_rule(&selector, &parent_indent, &unit, &cluster, source);
            let body = unwrap_style_block_body(&merged);
            let header = format!("@layer {}", path.join("."));
            layer_blocks.push((header, body));

            if layer_contains_only_selector_cluster(
                source,
                &hit.layer,
                &selector,
                &exact_homes,
                &home_weight,
            ) {
                if deleted_layers.insert(hit.layer.start) {
                    deletes.push((
                        hit.layer.start,
                        extend_with_trailing_newline(source, hit.layer.end),
                        hit.layer.start,
                    ));
                }
            } else {
                for member in &cluster {
                    match member {
                        GatherMember::Style(n) => {
                            let start = leading_block_comment(source, n.start)
                                .map(|(s, _)| s)
                                .unwrap_or(n.start);
                            deletes.push((
                                start,
                                extend_with_trailing_newline(source, n.end),
                                n.start,
                            ));
                        }
                        GatherMember::Conditional {
                            at_node,
                            inner,
                            delete_whole_at_block,
                        } => {
                            if *delete_whole_at_block {
                                deletes.push((
                                    at_node.start,
                                    extend_with_trailing_newline(source, at_node.end),
                                    at_node.start,
                                ));
                            } else {
                                deletes.push((
                                    inner.start,
                                    extend_with_trailing_newline(source, inner.end),
                                    inner.start,
                                ));
                            }
                        }
                    }
                }
            }
        }

        if layer_blocks.len() < 2 {
            continue;
        }

        let first_layer_only_ours = layer_contains_only_selector_cluster(
            source,
            first_layer,
            &selector,
            &exact_homes,
            &home_weight,
        );
        let insert_at = first_layer.start;
        let replace_end = if first_layer_only_ours {
            extend_with_trailing_newline(source, first_layer.end)
        } else {
            insert_at
        };
        deletes.retain(|(start, end, _)| !(*start == insert_at && *end == replace_end));
        let nested_indent = unit.clone();
        let inner_indent = format!("{unit}{unit}");
        let mut hoisted = format!("{selector} {{\n");
        for (i, (header, body)) in layer_blocks.iter().enumerate() {
            if i > 0 {
                hoisted.push('\n');
            }
            hoisted.push_str(&nested_indent);
            hoisted.push_str(header);
            hoisted.push_str(" {\n");
            if !body.trim().is_empty() {
                for line in reindent_owned(body, &inner_indent) {
                    hoisted.push_str(&line);
                    hoisted.push('\n');
                }
            }
            hoisted.push_str(&nested_indent);
            hoisted.push_str("}\n");
        }
        hoisted.push('}');
        if first_layer_only_ours {
            hoisted.push('\n');
        } else {
            hoisted.push('\n');
            hoisted.push('\n');
        }

        plans.push(PlanEntry {
            id: String::new(),
            file: path.to_path_buf(),
            rules: vec![RuleId::NestLayerBySelector],
            safety: Safety::Review,
            source_range: SourceRange {
                start: insert_at,
                end: replace_end,
            },
            original: if first_layer_only_ours {
                source[insert_at..replace_end].to_string()
            } else {
                String::new()
            },
            proposed: hoisted,
            proof: Proof {
                selector_set_equivalent: true,
                specificity_equivalent: true,
                cascade_context_equivalent: true,
                source_order_equivalent: false,
                layer_equivalent: true,
                scope_equivalent: true,
                declarations_exact: true,
                important_exact: true,
            },
            warnings: vec![format!(
                "Hoisted '{}' out of {} named layers so nested @layer blocks do not create child layers.",
                selector,
                layer_blocks.len()
            )],
            reason: format!(
                "Nest {} named layers under shared selector '{}'.",
                layer_blocks.len(),
                selector
            ),
            selected: true,
        });

        for (start, end, line_at) in deletes {
            plans.push(PlanEntry {
                id: String::new(),
                file: path.to_path_buf(),
                rules: vec![RuleId::NestLayerBySelector],
                safety: Safety::Review,
                source_range: SourceRange { start, end },
                original: source[start..end].to_string(),
                proposed: String::new(),
                proof: Proof::safe_local(),
                warnings: Vec::new(),
                reason: format!(
                    "Remove layer-local '{}' after nesting layers at line {}.",
                    selector,
                    line_number(source, line_at)
                ),
                selected: true,
            });
        }
    }
}

fn plan_gather_related_selector_rules(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::GatherRelatedSelectorRules) {
        return;
    }

    // This is a review-only advisory pass. Its home-assignment search is
    // quadratic in the number of distinct selectors, so bound it for large
    // flat stylesheets and keep the responsive, local transformations active.
    if nodes.len() > 256 {
        return;
    }

    let mut exact_homes: Vec<&str> = Vec::new();
    let mut home_weight: HashMap<&str, usize> = HashMap::new();
    collect_gather_homes(source, nodes, &mut exact_homes, &mut home_weight);

    let mut grouped_at_blocks: HashSet<usize> = HashSet::new();
    mark_grouped_at_blocks(
        source,
        nodes,
        &exact_homes,
        &home_weight,
        &mut grouped_at_blocks,
    );

    for base in exact_homes.clone() {
        // Global selector homes are too broad for structural gathering. They
        // can span most of a large stylesheet and win overlap resolution over
        // precise component plans, while also making it easy to move rules
        // across unrelated cascade contexts. Keep them as ordinary existing
        // rules; gather only under a concrete selector home.
        if is_weak_gather_base(base) {
            continue;
        }
        let mut cluster: Vec<GatherMember> = Vec::new();
        collect_gather_cluster(
            source,
            nodes,
            base,
            &exact_homes,
            &home_weight,
            &grouped_at_blocks,
            &mut cluster,
        );

        if cluster.len() > 1 {
            let mut is_non_adjacent = false;
            for window in cluster.windows(2) {
                let prev_end = window[0].outer_end();
                let next_start = window[1].outer_start();
                if next_start < prev_end {
                    continue;
                }
                if !is_whitespace_only(source, prev_end..next_start) {
                    is_non_adjacent = true;
                    break;
                }
            }

            let first_style = cluster.iter().find_map(|m| match m {
                GatherMember::Style(n) => Some(n),
                GatherMember::Conditional { .. } => None,
            });

            let Some(first) = first_style else {
                continue;
            };
            let first_sel = first.prelude(source).trim();
            let has_non_style = cluster
                .iter()
                .any(|m| matches!(m, GatherMember::Conditional { .. }));
            let should_gather = is_non_adjacent || first_sel != base || has_non_style;
            if !should_gather {
                continue;
            }

            let cluster_safe = cluster.iter().all(|member| {
                let sel = match member {
                    GatherMember::Style(n) => n.prelude(source).trim(),
                    GatherMember::Conditional { inner, .. } => inner.prelude(source).trim(),
                };
                can_nest_under_gather_home(base, sel)
            });
            if !cluster_safe {
                continue;
            }
            let parent_indent = line_indent(source, first.start);
            let unit = relative_indent_unit(source, first);

            let mut merged = format_merged_rule(base, &parent_indent, &unit, &cluster, source);

            let style_members: Vec<&SourceNode> = cluster
                .iter()
                .filter_map(|m| match m {
                    GatherMember::Style(n) => Some(n),
                    GatherMember::Conditional { .. } => None,
                })
                .collect();
            let container_end = enclosing_container_end(source, nodes, first);
            let (local_styles, remote_styles): (Vec<&SourceNode>, Vec<&SourceNode>) =
                style_members.iter().copied().partition(|sec| {
                    if let Some(limit) = container_end {
                        sec.end <= limit
                    } else {
                        nodes.iter().any(|n| n.start == sec.start)
                    }
                });
            let last_local = local_styles.last().copied().unwrap_or(first);
            let first_comment = leading_block_comment(source, first.start);
            let span_start =
                line_start(source, first_comment.map(|(s, _)| s).unwrap_or(first.start));
            let mut span_end = extend_with_trailing_newline(source, last_local.end);
            for member in &cluster {
                let GatherMember::Conditional {
                    at_node,
                    delete_whole_at_block,
                    ..
                } = member
                else {
                    continue;
                };
                if *delete_whole_at_block
                    && at_node.start >= span_start
                    && (container_end.is_none_or(|limit| at_node.end <= limit))
                {
                    span_end = span_end.max(extend_with_trailing_newline(source, at_node.end));
                }
            }
            if let Some((_, cmt)) = first_comment {
                merged = format!("{parent_indent}{cmt}\n{merged}");
            }

            let mut replacements = vec![(span_start, first.end, merged)];
            let mut cluster_starts = HashSet::new();
            for sec in &local_styles {
                cluster_starts.insert(sec.start);
            }
            for sec in local_styles.iter().skip(1) {
                let start = leading_block_comment(source, sec.start)
                    .map(|(s, _)| s)
                    .unwrap_or(sec.start);
                let end = extend_with_trailing_newline(source, sec.end);
                replacements.push((start, end, String::new()));
            }
            let mut replaced_at = HashSet::new();
            for member in &cluster {
                let GatherMember::Conditional {
                    at_node,
                    delete_whole_at_block,
                    ..
                } = member
                else {
                    continue;
                };
                if *delete_whole_at_block
                    && at_node.start >= span_start
                    && at_node.end <= span_end
                    && replaced_at.insert(at_node.start)
                {
                    replacements.push((
                        at_node.start,
                        extend_with_trailing_newline(source, at_node.end),
                        String::new(),
                    ));
                }
            }
            replacements.extend(leftover_style_replacements(
                source,
                nodes,
                span_start,
                span_end,
                &cluster_starts,
            ));
            let proposed = squeeze_excess_blank_lines(&apply_range_replacements(
                source,
                span_start,
                span_end,
                &replacements,
            ));

            plans.push(PlanEntry {
                id: String::new(),
                file: path.to_path_buf(),
                rules: vec![RuleId::GatherRelatedSelectorRules],
                safety: Safety::Review,
                source_range: SourceRange {
                    start: span_start,
                    end: span_end,
                },
                original: source[span_start..span_end].to_string(),
                proposed,
                proof: Proof {
                    selector_set_equivalent: true,
                    specificity_equivalent: true,
                    cascade_context_equivalent: false,
                    source_order_equivalent: false,
                    layer_equivalent: remote_styles.is_empty(),
                    scope_equivalent: true,
                    declarations_exact: true,
                    important_exact: true,
                },
                warnings: vec![format!(
                    "Gathered {} related occurrences of '{}' across lines; review cascade ordering.",
                    cluster.len(),
                    base
                )],
                reason: format!(
                    "Gather {} related rules for '{}' into the canonical first selector block.",
                    cluster.len(),
                    base
                ),
                selected: true,
            });

            let mut deleted_at_blocks = HashSet::new();
            for sec in &remote_styles {
                let start = leading_block_comment(source, sec.start)
                    .map(|(s, _)| s)
                    .unwrap_or(sec.start);
                let end = extend_with_trailing_newline(source, sec.end);
                plans.push(PlanEntry {
                    id: String::new(),
                    file: path.to_path_buf(),
                    rules: vec![RuleId::GatherRelatedSelectorRules],
                    safety: Safety::Review,
                    source_range: SourceRange { start, end },
                    original: source[start..end].to_string(),
                    proposed: String::new(),
                    proof: Proof::safe_local(),
                    warnings: Vec::new(),
                    reason: format!(
                        "Remove non-adjacent gathered rule for '{}' at line {}.",
                        base,
                        line_number(source, sec.start)
                    ),
                    selected: true,
                });
            }
            for member in &cluster {
                let GatherMember::Conditional {
                    at_node,
                    inner,
                    delete_whole_at_block,
                } = member
                else {
                    continue;
                };
                let (sec_start, sec_end, line_at) = if *delete_whole_at_block {
                    if !deleted_at_blocks.insert(at_node.start) {
                        continue;
                    }
                    (
                        at_node.start,
                        extend_with_trailing_newline(source, at_node.end),
                        at_node.start,
                    )
                } else {
                    (
                        inner.start,
                        extend_with_trailing_newline(source, inner.end),
                        inner.start,
                    )
                };

                plans.push(PlanEntry {
                    id: String::new(),
                    file: path.to_path_buf(),
                    rules: vec![RuleId::GatherRelatedSelectorRules],
                    safety: Safety::Review,
                    source_range: SourceRange {
                        start: sec_start,
                        end: sec_end,
                    },
                    original: source[sec_start..sec_end].to_string(),
                    proposed: String::new(),
                    proof: Proof::safe_local(),
                    warnings: Vec::new(),
                    reason: format!(
                        "Remove non-adjacent gathered rule for '{}' at line {}.",
                        base,
                        line_number(source, line_at)
                    ),
                    selected: true,
                });
            }
        }
    }
}

fn plan_factor_identical_states_with_is(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::FactorIdenticalStatesWithIs) {
        return;
    }
    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if matches!(&first.kind, NodeKind::Style) {
            let first_sel = first.prelude(source).trim();
            if let Some(colon_pos) = first_sel.find(':')
                && !first_sel[colon_pos..].starts_with("::")
            {
                let base = &first_sel[..colon_pos];
                if !base.is_empty() && !base.contains(' ') {
                    let first_body = first.body(source).unwrap_or("").trim();
                    let mut cluster = vec![first];
                    let mut cursor = i + 1;
                    let mut prev_end = first.end;

                    while cursor < nodes.len() {
                        let next = &nodes[cursor];
                        if !is_whitespace_only(source, prev_end..next.start) {
                            break;
                        }
                        if matches!(&next.kind, NodeKind::Style) {
                            let next_sel = next.prelude(source).trim();
                            if next_sel.starts_with(base)
                                && next_sel[base.len()..].starts_with(':')
                                && !next_sel[base.len()..].starts_with("::")
                                && next.body(source).unwrap_or("").trim() == first_body
                            {
                                cluster.push(next);
                                prev_end = next.end;
                                cursor += 1;
                                continue;
                            }
                        }
                        break;
                    }

                    if cluster.len() > 1 {
                        let last = cluster.last().unwrap();
                        let pseudos: Vec<&str> = cluster
                            .iter()
                            .map(|c| {
                                let s = c.prelude(source).trim();
                                &s[base.len()..]
                            })
                            .collect();
                        let is_inner = pseudos.join(", ");
                        let parent_indent = line_indent(source, first.start);
                        let first_body_range = first.body_range.as_ref().unwrap();
                        let unit = detect_indent_unit(source, first_body_range.clone())
                            .unwrap_or_else(|| "  ".to_string());
                        let nested_indent = format!("{parent_indent}{unit}");
                        let inner_decl_indent = format!("{nested_indent}{unit}");

                        let mut decls = String::new();
                        for line in source[first_body_range.clone()].lines() {
                            let trimmed = line.trim();
                            if !trimmed.is_empty() {
                                decls.push_str(&inner_decl_indent);
                                decls.push_str(&ensure_semicolon(trimmed));
                                decls.push('\n');
                            }
                        }

                        let proposed = format!(
                            "{parent_indent}{base} {{\n{nested_indent}&:is({is_inner}) {{\n{decls}{nested_indent}}}\n{parent_indent}}}"
                        );
                        plans.push(PlanEntry {
                            id: String::new(),
                            file: path.to_path_buf(),
                            rules: vec![RuleId::FactorIdenticalStatesWithIs],
                            safety: Safety::Safe,
                            source_range: SourceRange {
                                start: first.start,
                                end: last.end,
                            },
                            original: source[first.start..last.end].to_string(),
                            proposed,
                            proof: Proof::safe_local(),
                            warnings: Vec::new(),
                            reason: format!(
                                "Factor {} identical state rules for '{}' into &:is({}) form.",
                                cluster.len(),
                                base,
                                is_inner
                            ),
                            selected: true,
                        });
                        i = cursor;
                        continue;
                    }
                }
            }
        }
        i += 1;
    }
}

fn plan_factor_multi_selector_cluster_with_is(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::ModernizeIs) {
        return;
    }

    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if matches!(&first.kind, NodeKind::Style) {
            let first_sel = first.prelude(source).trim();
            if let Some((base_prefixes, first_suffix)) = extract_multi_branch_pattern(first_sel) {
                let mut cluster = vec![(first, first_suffix)];
                let mut cursor = i + 1;
                let mut prev_end = first.end;

                while cursor < nodes.len() {
                    let next = &nodes[cursor];
                    if !is_whitespace_only(source, prev_end..next.start) {
                        break;
                    }
                    if matches!(&next.kind, NodeKind::Style) {
                        let next_sel = next.prelude(source).trim();
                        if let Some((next_prefixes, next_suffix)) =
                            extract_multi_branch_pattern(next_sel)
                            && next_prefixes == base_prefixes
                        {
                            cluster.push((next, next_suffix));
                            prev_end = next.end;
                            cursor += 1;
                            continue;
                        }
                    }
                    break;
                }

                if cluster.len() > 1 {
                    let (last_node, _) = cluster.last().unwrap();
                    let parent_indent = line_indent(source, first.start);
                    let first_body_range = first.body_range.as_ref().unwrap();
                    let unit = detect_indent_unit(source, first_body_range.clone())
                        .unwrap_or_else(|| "  ".to_string());
                    let nested_indent = format!("{parent_indent}{unit}");
                    let inner_decl_indent = format!("{nested_indent}{unit}");

                    let is_header = format!(":is({})", base_prefixes.join(", "));
                    let mut out = format!("{parent_indent}{is_header} {{\n");
                    let mut has_direct_decls = false;

                    // 1. Direct declarations from base rules (suffix == None)
                    for &(c_node, ref suffix) in &cluster {
                        if suffix.is_none()
                            && let Some(c_body_range) = &c_node.body_range
                        {
                            for line in source[c_body_range.clone()].lines() {
                                let trimmed = line.trim();
                                if !trimmed.is_empty() {
                                    out.push_str(&nested_indent);
                                    out.push_str(&ensure_semicolon(trimmed));
                                    out.push('\n');
                                    has_direct_decls = true;
                                }
                            }
                        }
                    }

                    // 2. Nested child rules (suffix == Some(sub_sel))
                    for (c_idx, &(c_node, ref suffix)) in cluster.iter().enumerate() {
                        if let Some(sub_sel) = suffix {
                            if has_direct_decls || c_idx > 0 {
                                out.push('\n');
                            }
                            out.push_str(&nested_indent);
                            out.push_str(sub_sel);
                            out.push_str(" {\n");
                            if let Some(c_body_range) = &c_node.body_range {
                                for line in source[c_body_range.clone()].lines() {
                                    let trimmed = line.trim();
                                    if !trimmed.is_empty() {
                                        out.push_str(&inner_decl_indent);
                                        out.push_str(&ensure_semicolon(trimmed));
                                        out.push('\n');
                                    }
                                }
                            }
                            out.push_str(&nested_indent);
                            out.push_str("}\n");
                        }
                    }

                    out.push_str(&parent_indent);
                    out.push('}');

                    let specificities: Vec<Specificity> = base_prefixes
                        .iter()
                        .map(|p| calculate_specificity(p))
                        .collect();
                    let uniform = specificities.windows(2).all(|w| w[0] == w[1]);

                    plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::ModernizeIs],
                        safety: if uniform { Safety::Safe } else { Safety::Review },
                        source_range: SourceRange {
                            start: first.start,
                            end: last_node.end,
                        },
                        original: source[first.start..last_node.end].to_string(),
                        proposed: out,
                        proof: Proof {
                            specificity_equivalent: uniform,
                            ..Proof::safe_local()
                        },
                        warnings: if uniform {
                            Vec::new()
                        } else {
                            vec!["Mixed selector specificity: :is() takes the specificity of its most specific argument.".into()]
                        },
                        reason: format!("Factor multi-selector cluster for {} into :is(...) with nested rules.", is_header),
                        selected: true,
                    });
                    i = cursor;
                    continue;
                }
            }
        }
        i += 1;
    }
}

fn extract_multi_branch_pattern(selector: &str) -> Option<(Vec<String>, Option<String>)> {
    let branches: Vec<&str> = split_top_level_comma(selector)
        .into_iter()
        .map(|s| s.trim())
        .collect();
    if branches.len() < 2 {
        return None;
    }
    if branches.iter().any(|b| b.contains("::")) {
        return None;
    }

    let first = branches[0];
    if let Some(space_pos) = first.rfind(' ') {
        let suffix = &first[space_pos..];
        if branches.iter().all(|b| b.ends_with(suffix)) {
            let prefixes: Vec<String> = branches
                .iter()
                .map(|b| b[..b.len() - suffix.len()].trim().to_string())
                .collect();
            if prefixes.iter().all(|p| is_valid_selector_token(p)) {
                return Some((prefixes, Some(suffix.trim().to_string())));
            }
        }
    }

    if branches
        .iter()
        .all(|b| is_valid_selector_token(b) && !b.contains(' '))
    {
        let prefixes: Vec<String> = branches.iter().map(|b| b.to_string()).collect();
        return Some((prefixes, None));
    }

    None
}

fn plan_merge_identical_rule_bodies(
    path: &Path,
    source: &str,
    nodes: &[SourceNode],
    enabled: &HashSet<RuleId>,
    plans: &mut Vec<PlanEntry>,
) {
    if !enabled.contains(&RuleId::MergeIdenticalRuleBodies) {
        return;
    }
    let mut i = 0;
    while i < nodes.len() {
        let first = &nodes[i];
        if matches!(&first.kind, NodeKind::Style) {
            let first_body = first.body(source).unwrap_or("").trim();
            if !first_body.is_empty() {
                let mut cluster = vec![first];
                let mut cursor = i + 1;
                let mut prev_end = first.end;

                while cursor < nodes.len() {
                    let next = &nodes[cursor];
                    if !is_whitespace_only(source, prev_end..next.start) {
                        break;
                    }
                    if matches!(&next.kind, NodeKind::Style)
                        && next.body(source).unwrap_or("").trim() == first_body
                    {
                        cluster.push(next);
                        prev_end = next.end;
                        cursor += 1;
                        continue;
                    }
                    break;
                }

                if cluster.len() > 1 {
                    let last = cluster.last().unwrap();
                    let selectors: Vec<&str> =
                        cluster.iter().map(|c| c.prelude(source).trim()).collect();
                    let specificities: Vec<Specificity> =
                        selectors.iter().map(|s| calculate_specificity(s)).collect();
                    let uniform_specificity = specificities.windows(2).all(|w| w[0] == w[1]);
                    let parent_indent = line_indent(source, first.start);
                    let first_body_range = first.body_range.as_ref().unwrap();
                    let unit = detect_indent_unit(source, first_body_range.clone())
                        .unwrap_or_else(|| "  ".to_string());
                    let nested_indent = format!("{parent_indent}{unit}");

                    let mut decls = String::new();
                    for line in source[first_body_range.clone()].lines() {
                        let trimmed = line.trim();
                        if !trimmed.is_empty() {
                            decls.push_str(&nested_indent);
                            decls.push_str(&ensure_semicolon(trimmed));
                            decls.push('\n');
                        }
                    }

                    let joined_sel = selectors.join(&format!(",\n{parent_indent}"));
                    let proposed =
                        format!("{parent_indent}{joined_sel} {{\n{decls}{parent_indent}}}");
                    plans.push(PlanEntry {
                        id: String::new(),
                        file: path.to_path_buf(),
                        rules: vec![RuleId::MergeIdenticalRuleBodies],
                        safety: if uniform_specificity {
                            Safety::Safe
                        } else {
                            Safety::Review
                        },
                        source_range: SourceRange {
                            start: first.start,
                            end: last.end,
                        },
                        original: source[first.start..last.end].to_string(),
                        proposed,
                        proof: Proof {
                            specificity_equivalent: uniform_specificity,
                            ..Proof::safe_local()
                        },
                        warnings: if uniform_specificity {
                            Vec::new()
                        } else {
                            vec!["Mixed selector specificity: combining these selectors changes cascade weight.".into()]
                        },
                        reason: format!("Merge {} rules with identical declaration bodies into a single comma-separated rule.", cluster.len()),
                        selected: true,
                    });
                    i = cursor;
                    continue;
                }
            }
        }
        i += 1;
    }
}

pub fn split_top_level_comma(selector: &str) -> Vec<&str> {
    let bytes = selector.as_bytes();
    let mut parts = Vec::new();
    let mut last = 0;
    let mut parens = 0usize;
    let mut brackets = 0usize;
    let mut quote: Option<u8> = None;
    let mut escaped = false;
    let mut i = 0usize;

    while i < bytes.len() {
        let b = bytes[i];
        if let Some(q) = quote {
            if escaped {
                escaped = false;
            } else if b == b'\\' {
                escaped = true;
            } else if b == q {
                quote = None;
            }
            i += 1;
            continue;
        }
        match b {
            b'\'' | b'"' => quote = Some(b),
            b'(' => parens += 1,
            b')' => parens = parens.saturating_sub(1),
            b'[' => brackets += 1,
            b']' => brackets = brackets.saturating_sub(1),
            b',' if parens == 0 && brackets == 0 => {
                parts.push(&selector[last..i]);
                last = i + 1;
            }
            _ => {}
        }
        i += 1;
    }
    if last < selector.len() {
        parts.push(&selector[last..]);
    }
    parts
}

pub fn factor_selector_list(
    selector: &str,
    body: &str,
    indent: &str,
    unit: &str,
) -> Option<String> {
    let branches: Vec<&str> = split_top_level_comma(selector)
        .into_iter()
        .map(|s| s.trim())
        .collect();
    if branches.len() < 2 {
        return None;
    }
    let base = branches[0];
    if contains_top_level_comma(base) || base.contains("::") || base.is_empty() {
        return None;
    }

    let mut inner_selectors = Vec::new();
    for &branch in &branches {
        if branch == base {
            inner_selectors.push("&".to_string());
        } else {
            let rel = branch.strip_prefix(base)?;
            if rel.starts_with("::")
                || rel.starts_with(':')
                || rel.starts_with('[')
                || rel.starts_with('.')
                || rel.starts_with('#')
            {
                inner_selectors.push(format!("&{rel}"));
            } else {
                let trimmed = rel.strip_prefix(' ')?;
                inner_selectors.push(trimmed.trim_start().to_string());
            }
        }
    }

    let nested_indent = format!("{indent}{unit}");
    let inner_decl_indent = format!("{nested_indent}{unit}");
    let mut out = String::new();
    out.push_str(base);
    out.push_str(" {\n");
    out.push_str(&nested_indent);
    out.push_str(&inner_selectors.join(&format!(",\n{nested_indent}")));
    out.push_str(" {\n");
    for line in preserve_body_lines(body) {
        out.push_str(&inner_decl_indent);
        out.push_str(&ensure_semicolon(&line));
        out.push('\n');
    }
    out.push_str(&nested_indent);
    out.push_str("}\n");
    out.push_str(indent);
    out.push('}');
    Some(out)
}

/// Returns the first functional pseudo-class call that occurs at selector
/// level, keeping the text before, inside, and after the call separate.
/// Parentheses and quoted strings inside the argument list are respected.
fn split_outer_function_call(selector: &str) -> Option<(&str, &'static str, &str, &str)> {
    let functions = [
        (":is(", "is"),
        (":where(", "where"),
        (":has(", "has"),
        (":not(", "not"),
    ];
    let bytes = selector.as_bytes();
    let mut depth = 0usize;
    let mut quote = None;
    let mut escaped = false;
    let mut i = 0usize;

    while i < bytes.len() {
        let b = bytes[i];
        if let Some(q) = quote {
            if escaped {
                escaped = false;
            } else if b == b'\\' {
                escaped = true;
            } else if b == q {
                quote = None;
            }
            i += 1;
            continue;
        }
        if b == b'\'' || b == b'"' {
            quote = Some(b);
            i += 1;
            continue;
        }
        if b == b'(' {
            depth += 1;
            i += 1;
            continue;
        }
        if b == b')' {
            depth = depth.saturating_sub(1);
            i += 1;
            continue;
        }
        if depth == 0 {
            for (needle, name) in functions {
                if selector[i..].starts_with(needle) {
                    let open = i + needle.len() - 1;
                    let close = find_matching_paren(selector, open)?;
                    return Some((
                        &selector[..i],
                        name,
                        &selector[open + 1..close],
                        &selector[close + 1..],
                    ));
                }
            }
        }
        i += 1;
    }
    None
}

fn factor_outer_function_arguments(
    branches: &[&str],
    uniform_specificity: bool,
) -> Option<(String, bool)> {
    let (first_prefix, first_name, first_args, first_suffix) =
        split_outer_function_call(branches[0])?;
    // Separate :not() rules are an OR of negations; putting their arguments
    // into one :not() changes that to an intersection and is not equivalent.
    if first_name == "not" || first_args.trim().is_empty() {
        return None;
    }

    let mut arguments = vec![first_args.trim()];
    for branch in &branches[1..] {
        let (prefix, name, args, suffix) = split_outer_function_call(branch)?;
        if prefix != first_prefix
            || name != first_name
            || suffix != first_suffix
            || args.trim().is_empty()
        {
            return None;
        }
        arguments.push(args.trim());
    }

    let combined = format!(
        "{first_prefix}:{first_name}({}){first_suffix}",
        arguments.join(", ")
    );
    // :where() contributes zero specificity regardless of its arguments.
    let safe = first_name == "where" || uniform_specificity;
    Some((combined, safe))
}

fn contains_pseudo_element_syntax(selector: &str) -> bool {
    if selector.contains("::") {
        return true;
    }

    // CSS2 permits the legacy single-colon spelling only for these
    // pseudo-elements. They are still pseudo-elements, so putting them in
    // :is(), :where(), or :not() is invalid even though they use one colon.
    const LEGACY_PSEUDO_ELEMENTS: &[&str] = &[":before", ":after", ":first-line", ":first-letter"];
    LEGACY_PSEUDO_ELEMENTS.iter().any(|pseudo| {
        let mut offset = 0;
        while let Some(relative) = selector[offset..].find(pseudo) {
            let start = offset + relative;
            let end = start + pseudo.len();
            let boundary = selector[end..].chars().next();
            if boundary.is_none_or(|c| !c.is_ascii_alphanumeric() && c != '-' && c != '_') {
                return true;
            }
            offset = end;
        }
        false
    })
}

pub fn factor_with_is(selector: &str) -> Option<(String, bool)> {
    let branches: Vec<&str> = split_top_level_comma(selector)
        .into_iter()
        .map(|s| s.trim())
        .filter(|s| !s.is_empty())
        .collect();
    if branches.len() < 2 {
        return None;
    }

    if branches.iter().any(|b| contains_pseudo_element_syntax(b)) {
        return None;
    }

    let specificities: Vec<Specificity> =
        branches.iter().map(|b| calculate_specificity(b)).collect();
    let uniform_specificity = specificities.windows(2).all(|w| w[0] == w[1]);

    if let Some(result) = factor_outer_function_arguments(&branches, uniform_specificity) {
        return Some(result);
    }

    // The remaining factoring patterns operate on top-level selector
    // boundaries. Do not let them inspect commas or combinators inside a
    // functional pseudo-class, where they could produce malformed output.
    if branches.iter().any(|branch| {
        [":is(", ":where(", ":has(", ":not("]
            .iter()
            .any(|function| branch.contains(function))
    }) {
        return None;
    }

    let first = branches[0];

    // Suffix alternatives (e.g. .alpha .title, #hero .title -> :is(.alpha, #hero) .title)
    if let Some(space_pos) = first.rfind(' ') {
        let suffix = &first[space_pos..];
        if branches.iter().all(|b| b.ends_with(suffix)) {
            let prefixes: Vec<&str> = branches
                .iter()
                .map(|b| b[..b.len() - suffix.len()].trim())
                .collect();
            if prefixes.iter().all(|p| is_valid_selector_token(p)) {
                let is_inner = prefixes.join(", ");
                return Some((format!(":is({is_inner}){suffix}"), uniform_specificity));
            }
        }
    }

    // Descendant alternatives (e.g. .card .title, .card .subtitle -> .card :is(.title, .subtitle))
    if let Some(space_pos) = first.rfind(' ') {
        let prefix = &first[..=space_pos];
        if branches.iter().all(|b| b.starts_with(prefix)) {
            let suffixes: Vec<&str> = branches.iter().map(|b| b[prefix.len()..].trim()).collect();
            if suffixes.iter().all(|s| is_valid_selector_token(s)) {
                let is_inner = suffixes.join(", ");
                return Some((format!("{prefix}:is({is_inner})"), uniform_specificity));
            }
        }
    }

    // Pseudo-class alternatives (e.g. .button:hover, .button:focus -> .button:is(:hover, :focus))
    if let Some(colon_pos) = first.find(':') {
        let base = &first[..colon_pos];
        if !base.is_empty()
            && !base.contains(' ')
            && branches.iter().all(|b| {
                b.starts_with(base)
                    && b[base.len()..].starts_with(':')
                    && !b[base.len()..].starts_with("::")
            })
        {
            let pseudos: Vec<&str> = branches.iter().map(|b| b[base.len()..].trim()).collect();
            if pseudos
                .iter()
                .all(|p| p.starts_with(':') && !p.starts_with("::") && !p.contains(' '))
            {
                let is_inner = pseudos.join(", ");
                return Some((format!("{base}:is({is_inner})"), uniform_specificity));
            }
        }
    }

    // Attribute alternatives
    if let Some(bracket_pos) = first.find('[') {
        let base = &first[..bracket_pos];
        if !base.is_empty()
            && !base.contains(' ')
            && branches
                .iter()
                .all(|b| b.starts_with(base) && b[base.len()..].starts_with('['))
        {
            let attrs: Vec<&str> = branches.iter().map(|b| b[base.len()..].trim()).collect();
            if attrs.iter().all(|a| a.starts_with('[') && a.ends_with(']')) {
                let is_inner = attrs.join(", ");
                return Some((format!("{base}:is({is_inner})"), uniform_specificity));
            }
        }
    }

    None
}

fn is_valid_selector_token(s: &str) -> bool {
    if s.is_empty() {
        return false;
    }
    let first = s.chars().next().unwrap();
    first == '.'
        || first == '#'
        || first == '['
        || first == ':'
        || first.is_ascii_alphabetic()
        || first == '*'
        || first == '>'
        || first == '+'
        || first == '~'
}

pub fn factor_with_where(selector: &str) -> Option<String> {
    let branches: Vec<&str> = split_top_level_comma(selector)
        .into_iter()
        .map(|s| s.trim())
        .collect();
    if branches.len() < 2 {
        return None;
    }
    if branches.iter().any(|b| b.contains("::")) {
        return None;
    }
    Some(format!(":where({})", branches.join(", ")))
}

pub fn modernize_media_query_str(prelude: &str) -> Option<String> {
    let mut result = prelude.to_string();
    let mut changed = false;

    // Range: (min-width: 400px) and (max-width: 800px) -> (400px <= width <= 800px)
    if let (Some((min_raw, min_val)), Some((max_raw, max_val))) = (
        extract_media_feature_and_raw(&result, "min-width"),
        extract_media_feature_and_raw(&result, "max-width"),
    ) {
        let pattern = format!("{min_raw} and {max_raw}");
        let replacement = format!("({min_val} <= width <= {max_val})");
        if result.contains(&pattern) {
            result = result.replace(&pattern, &replacement);
            changed = true;
        }
    }

    // Single: (min-width: 800px) -> (width >= 800px)
    while let Some((raw, val)) = extract_media_feature_and_raw(&result, "min-width") {
        let replacement = format!("(width >= {val})");
        result = result.replacen(&raw, &replacement, 1);
        changed = true;
    }

    // Single: (max-width: 800px) -> (width <= 800px)
    while let Some((raw, val)) = extract_media_feature_and_raw(&result, "max-width") {
        let replacement = format!("(width <= {val})");
        result = result.replacen(&raw, &replacement, 1);
        changed = true;
    }

    // Single: (min-height: 400px) -> (height >= 400px)
    while let Some((raw, val)) = extract_media_feature_and_raw(&result, "min-height") {
        let replacement = format!("(height >= {val})");
        result = result.replacen(&raw, &replacement, 1);
        changed = true;
    }

    // Single: (max-height: 400px) -> (height <= 400px)
    while let Some((raw, val)) = extract_media_feature_and_raw(&result, "max-height") {
        let replacement = format!("(height <= {val})");
        result = result.replacen(&raw, &replacement, 1);
        changed = true;
    }

    if changed { Some(result) } else { None }
}

fn extract_media_feature_and_raw(source: &str, feature: &str) -> Option<(String, String)> {
    let feat_idx = source.find(feature)?;
    let open_paren = source[..feat_idx].rfind('(')?;
    if source[open_paren..feat_idx].contains(')') {
        return None;
    }
    let colon_rel = source[feat_idx + feature.len()..].find(':')?;
    let colon_idx = feat_idx + feature.len() + colon_rel;
    let close_paren_rel = source[colon_idx..].find(')')?;
    let close_paren = colon_idx + close_paren_rel;

    let raw = source[open_paren..=close_paren].to_string();
    let val = source[colon_idx + 1..close_paren].trim().to_string();
    Some((raw, val))
}

fn selector_relation(parent: &str, child: &str) -> Option<(RelationKind, String)> {
    let parent = parent.trim();
    let child = child.trim();
    if parent.is_empty()
        || child.is_empty()
        || contains_top_level_comma(parent)
        || contains_top_level_comma(child)
        || parent.contains("::")
        || child == parent
        || selector_contains_nesting_amp(child)
    {
        return None;
    }

    if !child.starts_with(parent) {
        if is_weak_gather_base(parent) {
            return None;
        }
        return appended_selector_relation(parent, child);
    }

    let remainder = &child[parent.len()..];
    let trimmed = remainder.trim_start();
    if trimmed.is_empty() {
        return None;
    }

    let (relation, nested_selector) = if remainder.starts_with("::") {
        (RelationKind::PseudoElement, format!("&{remainder}"))
    } else if remainder.starts_with(':') {
        (RelationKind::PseudoClass, format!("&{remainder}"))
    } else if remainder.starts_with('[') {
        (RelationKind::Attribute, format!("&{remainder}"))
    } else if remainder.starts_with('.') || remainder.starts_with('#') {
        (RelationKind::Compound, format!("&{remainder}"))
    } else if let Some(first_char) = trimmed
        .chars()
        .next()
        .filter(|c| *c == '>' || *c == '+' || *c == '~')
    {
        let after_comb = trimmed[first_char.len_utf8()..].trim();
        if after_comb == parent {
            (
                RelationKind::Combinator,
                explicit_relative_combinator(first_char, parent),
            )
        } else if let Some(after_parent) = after_comb.strip_prefix(parent) {
            (
                RelationKind::Combinator,
                explicit_relative_combinator(first_char, &format!("{parent}{after_parent}")),
            )
        } else {
            (
                RelationKind::Combinator,
                explicit_relative_combinator(first_char, after_comb),
            )
        }
    } else if remainder
        .as_bytes()
        .first()
        .is_some_and(|b| b.is_ascii_whitespace())
    {
        let descendant = remainder.trim();
        (RelationKind::Descendant, descendant.to_string())
    } else {
        return None;
    };

    Some((relation, nested_selector))
}

fn conditional_child(
    source: &str,
    parent_selector: &str,
    node: &SourceNode,
    enabled: &HashSet<RuleId>,
) -> Option<ClusterChild> {
    let (name, rule) = match &node.kind {
        NodeKind::AtBlock { name, .. } if name == "media" => (name.as_str(), RuleId::NestMedia),
        NodeKind::AtBlock { name, .. } if name == "supports" => {
            (name.as_str(), RuleId::NestSupports)
        }
        NodeKind::AtBlock { name, .. } if name == "container" => {
            (name.as_str(), RuleId::NestContainer)
        }
        NodeKind::AtBlock { name, .. } if name == "starting-style" => {
            (name.as_str(), RuleId::NestStartingStyle)
        }
        _ => return None,
    };
    if !enabled.contains(&rule) {
        return None;
    }

    let body_range = node.body_range.clone()?;
    let inner_nodes = scan_nodes(source, body_range.clone());
    if inner_nodes.is_empty() {
        return None;
    }

    let mut inners = Vec::new();
    for inner in &inner_nodes {
        if !matches!(&inner.kind, NodeKind::Style) {
            return None;
        }
        let inner_prelude = inner.prelude(source);
        let inner_body = inner.body_range.clone()?;
        if inner_prelude == parent_selector.trim() {
            inners.push(ConditionalInner::Direct {
                body_range: inner_body,
            });
        } else if name != "starting-style"
            && let Some((_rel, nested_sel)) = selector_relation(parent_selector, inner_prelude)
        {
            inners.push(ConditionalInner::Nested {
                nested_selector: nested_sel,
                body_range: inner_body,
            });
        } else {
            return None;
        }
    }

    debug_assert!(
        name == "media" || name == "supports" || name == "container" || name == "starting-style"
    );
    Some(ClusterChild::Conditional {
        node: node.clone(),
        rule,
        inners,
    })
}

pub fn consolidate_not_in_selector(selector: &str) -> Option<(String, bool)> {
    if !selector.contains(":not(") {
        return None;
    }
    let mut result = String::new();
    let mut i = 0;
    let bytes = selector.as_bytes();
    let mut changed = false;
    let mut uniform_specificity = true;

    while i < bytes.len() {
        if i + 5 <= bytes.len() && &selector[i..i + 5] == ":not(" {
            let mut args = Vec::new();
            let mut current_end = i;

            while current_end + 5 <= bytes.len()
                && &selector[current_end..current_end + 5] == ":not("
            {
                let open = current_end + 4;
                if let Some(close) = find_matching_paren(selector, open) {
                    let arg = selector[open + 1..close].trim();
                    args.push(arg);
                    current_end = close + 1;
                } else {
                    break;
                }
            }

            if args.len() > 1 {
                changed = true;
                let specs: Vec<Specificity> =
                    args.iter().map(|a| calculate_specificity(a)).collect();
                if specs.windows(2).any(|w| w[0] != w[1]) {
                    uniform_specificity = false;
                }

                result.push_str(":not(");
                result.push_str(&args.join(", "));
                result.push(')');
                i = current_end;
                continue;
            }
        }
        let ch = selector[i..].chars().next().unwrap();
        result.push(ch);
        i += ch.len_utf8();
    }

    if changed {
        Some((result, uniform_specificity))
    } else {
        None
    }
}

fn find_matching_paren(source: &str, open: usize) -> Option<usize> {
    let bytes = source.as_bytes();
    let mut depth = 1usize;
    let mut i = open + 1;
    let mut quote: Option<u8> = None;
    let mut escaped = false;

    while i < bytes.len() {
        let b = bytes[i];
        if let Some(q) = quote {
            if escaped {
                escaped = false;
            } else if b == b'\\' {
                escaped = true;
            } else if b == q {
                quote = None;
            }
            i += 1;
            continue;
        }

        match b {
            b'\'' | b'"' => quote = Some(b),
            b'(' => depth += 1,
            b')' => {
                depth -= 1;
                if depth == 0 {
                    return Some(i);
                }
            }
            _ => {}
        }
        i += 1;
    }
    None
}

#[derive(Debug, Clone)]
struct HierarchicalRule {
    relative_selector: String,
    body_lines: Vec<String>,
    sub_rules: Vec<HierarchicalRule>,
    conditional_header: Option<String>,
}

fn preserve_body_lines(body: &str) -> Vec<String> {
    let base_indent = body
        .lines()
        .find(|line| !line.trim().is_empty())
        .map(|line| line.len() - line.trim_start().len())
        .unwrap_or(0);
    body.lines()
        .filter(|line| !line.trim().is_empty())
        .map(|line| {
            line.get(base_indent..)
                .unwrap_or(line)
                .trim_end()
                .to_string()
        })
        .collect()
}

fn render_cluster(source: &str, parent: &SourceNode, children: &[ClusterChild]) -> String {
    let parent_body_range = parent.body_range.as_ref().expect("style rules have bodies");
    let parent_indent = line_indent(source, parent.start);
    let unit = relative_indent_unit(source, parent);
    let nested_indent = format!("{parent_indent}{unit}");

    let mut out = String::new();
    let open = parent_body_range.start - 1;
    out.push_str(&source[parent.start..=open]);

    let parent_body = &source[parent_body_range.clone()];
    let trimmed_body = parent_body.trim();
    if !trimmed_body.is_empty() {
        out.push('\n');
        for line in preserve_body_lines(parent_body) {
            out.push_str(&nested_indent);
            out.push_str(&ensure_semicolon(&line));
            out.push('\n');
        }
    }

    // Build hierarchical rules
    let mut root_rules: Vec<HierarchicalRule> = Vec::new();

    for child in children {
        match child {
            ClusterChild::Style {
                node,
                nested_selector,
                ..
            } => {
                let mut body_lines = Vec::new();
                if let Some(body_range) = &node.body_range {
                    body_lines = preserve_body_lines(&source[body_range.clone()]);
                }
                insert_hierarchical_style(&mut root_rules, nested_selector.trim(), body_lines);
            }
            ClusterChild::Conditional { node, inners, .. } => {
                let header = node.prelude(source).trim().to_string();
                let mut cond_sub_rules = Vec::new();
                for inner in inners {
                    match inner {
                        ConditionalInner::Direct { body_range } => {
                            let lines = preserve_body_lines(&source[body_range.clone()]);
                            cond_sub_rules.push(HierarchicalRule {
                                relative_selector: String::new(),
                                body_lines: lines,
                                sub_rules: Vec::new(),
                                conditional_header: None,
                            });
                        }
                        ConditionalInner::Nested {
                            nested_selector,
                            body_range,
                        } => {
                            let lines = preserve_body_lines(&source[body_range.clone()]);
                            cond_sub_rules.push(HierarchicalRule {
                                relative_selector: nested_selector.trim().to_string(),
                                body_lines: lines,
                                sub_rules: Vec::new(),
                                conditional_header: None,
                            });
                        }
                    }
                }
                root_rules.push(HierarchicalRule {
                    relative_selector: String::new(),
                    body_lines: Vec::new(),
                    sub_rules: cond_sub_rules,
                    conditional_header: Some(header),
                });
            }
        }
    }

    for rule in &root_rules {
        out.push('\n');
        render_hierarchical_rule(&mut out, rule, &nested_indent, &unit);
    }

    out.push_str(&parent_indent);
    out.push('}');
    out
}

fn insert_hierarchical_style(
    root_rules: &mut Vec<HierarchicalRule>,
    selector: &str,
    body_lines: Vec<String>,
) {
    if let Some(last_rule) = root_rules.last_mut()
        && last_rule.conditional_header.is_none()
        && !last_rule.relative_selector.is_empty()
    {
        let parent_sel = &last_rule.relative_selector;
        if let Some(rel) = extract_relative_subselector(parent_sel, selector) {
            insert_hierarchical_style(&mut last_rule.sub_rules, &rel, body_lines);
            return;
        }
    }

    root_rules.push(HierarchicalRule {
        relative_selector: selector.to_string(),
        body_lines,
        sub_rules: Vec::new(),
        conditional_header: None,
    });
}

fn extract_relative_subselector(parent: &str, child: &str) -> Option<String> {
    let parent = parent.trim();
    let child = child.trim();
    if child == parent || !child.starts_with(parent) {
        return None;
    }
    let remainder = &child[parent.len()..];
    let trimmed = remainder.trim_start();
    if trimmed.is_empty() {
        return None;
    }

    if remainder.starts_with("::")
        || remainder.starts_with(':')
        || remainder.starts_with('[')
        || remainder.starts_with('.')
        || remainder.starts_with('#')
    {
        Some(format!("&{remainder}"))
    } else if let Some(first_char) = trimmed
        .chars()
        .next()
        .filter(|c| *c == '>' || *c == '+' || *c == '~')
    {
        let after_comb = trimmed[first_char.len_utf8()..].trim();
        Some(explicit_relative_combinator(first_char, after_comb))
    } else if remainder
        .as_bytes()
        .first()
        .is_some_and(|b| b.is_ascii_whitespace())
    {
        Some(trimmed.to_string())
    } else {
        None
    }
}

fn render_hierarchical_rule(out: &mut String, rule: &HierarchicalRule, indent: &str, unit: &str) {
    let inner_indent = format!("{indent}{unit}");

    if let Some(header) = &rule.conditional_header {
        out.push_str(indent);
        out.push_str(header);
        out.push_str(" {\n");
        for (idx, sub) in rule.sub_rules.iter().enumerate() {
            if idx > 0 {
                out.push('\n');
            }
            if sub.relative_selector.is_empty() {
                for line in &sub.body_lines {
                    out.push_str(&inner_indent);
                    out.push_str(&ensure_semicolon(line));
                    out.push('\n');
                }
            } else {
                render_hierarchical_rule(out, sub, &inner_indent, unit);
            }
        }
        out.push_str(indent);
        out.push_str("}\n");
    } else {
        out.push_str(indent);
        out.push_str(&rule.relative_selector);
        out.push_str(" {\n");

        for line in &rule.body_lines {
            out.push_str(&inner_indent);
            out.push_str(&ensure_semicolon(line));
            out.push('\n');
        }

        for sub in &rule.sub_rules {
            out.push('\n');
            render_hierarchical_rule(out, sub, &inner_indent, unit);
        }

        out.push_str(indent);
        out.push_str("}\n");
    }
}

fn line_indent(source: &str, offset: usize) -> String {
    let line_start = source[..offset].rfind('\n').map_or(0, |idx| idx + 1);
    source[line_start..offset]
        .chars()
        .take_while(|c| c.is_whitespace() && *c != '\n' && *c != '\r')
        .collect()
}

/// Ensure a CSS declaration line ends with `;`.
/// Only adds the semicolon when the line looks like a property declaration
/// (contains `:`, does not already end with `;`, `{`, or `}`, and is not a comment).
fn ensure_semicolon(line: &str) -> std::borrow::Cow<'_, str> {
    let trimmed = line.trim_end();
    if trimmed.ends_with(';')
        || trimmed.ends_with('{')
        || trimmed.ends_with('}')
        || trimmed.ends_with(',')
        || trimmed.ends_with(':')
        || trimmed.starts_with("//")
        || trimmed.starts_with("/*")
        || !trimmed.contains(':')
    {
        return std::borrow::Cow::Borrowed(line);
    }
    std::borrow::Cow::Owned(format!("{trimmed};"))
}

fn line_number(source: &str, offset: usize) -> usize {
    source[..offset.min(source.len())].lines().count()
}

fn detect_indent_unit(source: &str, body: Range<usize>) -> Option<String> {
    for line in source[body].lines() {
        if line.trim().is_empty() {
            continue;
        }
        let indent: String = line
            .chars()
            .take_while(|c| *c == ' ' || *c == '\t')
            .collect();
        if !indent.is_empty() {
            return Some(indent);
        }
    }
    None
}

fn contains_top_level_comma(selector: &str) -> bool {
    let bytes = selector.as_bytes();
    let mut parens = 0usize;
    let mut brackets = 0usize;
    let mut quote: Option<u8> = None;
    let mut escaped = false;
    let mut i = 0usize;
    while i < bytes.len() {
        let b = bytes[i];
        if let Some(q) = quote {
            if escaped {
                escaped = false;
            } else if b == b'\\' {
                escaped = true;
            } else if b == q {
                quote = None;
            }
            i += 1;
            continue;
        }
        match b {
            b'\'' | b'"' => quote = Some(b),
            b'(' => parens += 1,
            b')' => parens = parens.saturating_sub(1),
            b'[' => brackets += 1,
            b']' => brackets = brackets.saturating_sub(1),
            b',' if parens == 0 && brackets == 0 => return true,
            _ => {}
        }
        i += 1;
    }
    false
}

pub fn apply_selected_plans(
    source: &str,
    plans: &[PlanEntry],
    include_review: bool,
) -> Result<String> {
    let selected: Vec<&PlanEntry> = plans
        .iter()
        .filter(|plan| {
            plan.selected
                && (plan.safety == Safety::Safe
                    || (include_review && plan.safety == Safety::Review))
        })
        .collect();
    let owned: Vec<PlanEntry> = selected.into_iter().cloned().collect();
    let keep = select_disjoint_plan_indices(&owned);
    let mut non_overlapping: Vec<&PlanEntry> = keep.iter().map(|&i| &owned[i]).collect();
    non_overlapping.sort_by(|a, b| {
        a.source_range
            .start
            .cmp(&b.source_range.start)
            .then_with(|| b.source_range.end.cmp(&a.source_range.end))
    });

    let mut output = source.to_string();
    for plan in non_overlapping.into_iter().rev() {
        if plan.source_range.start <= output.len()
            && plan.source_range.end <= output.len()
            && plan.source_range.start <= plan.source_range.end
        {
            output.replace_range(
                plan.source_range.start..plan.source_range.end,
                &plan.proposed,
            );
        }
    }
    Ok(output)
}

/// Apply all currently available safe/review plans until the source reaches a
/// fixed point. Plans can overlap, so one disjoint pass may intentionally leave
/// a second transformation for the next analysis pass.
pub fn apply_until_stable(
    path: &Path,
    source: &str,
    enabled_rules: &[RuleId],
    include_review: bool,
) -> Result<String> {
    let mut current = source.to_string();

    for _ in 0..32 {
        let report = analyze_content(path.to_path_buf(), &current, enabled_rules)?;
        let mut plans = report.plans;
        for plan in &mut plans {
            plan.selected =
                plan.safety == Safety::Safe || (include_review && plan.safety == Safety::Review);
        }

        let next = apply_selected_plans(&current, &plans, include_review)?;
        if next == current {
            return Ok(current);
        }
        current = next;
    }

    anyhow::bail!(
        "CSS transformations did not reach a stable result after 32 passes for {}",
        path.display()
    )
}

pub fn unified_diff(old: &str, new: &str, old_name: &str, new_name: &str) -> String {
    TextDiff::from_lines(old, new)
        .unified_diff()
        .header(old_name, new_name)
        .to_string()
}

#[cfg(test)]
mod tests {
    use super::*;

    fn plan(css: &str, rules: &[RuleId]) -> Vec<PlanEntry> {
        analyze_source(PathBuf::from("test.css"), css, rules)
            .unwrap()
            .plans
    }

    #[test]
    fn extracts_style_blocks_from_template_sources() {
        let source = r#"@php($title = 'demo')
<div>
  <style scoped>
    .card { color: red; }
  </style>
  <style>.badge { color: blue; }</style>
</div>
"#;
        let blocks = extract_style_blocks(source);
        assert_eq!(blocks.len(), 2);
        assert_eq!(
            &source[blocks[0].clone()],
            "\n    .card { color: red; }\n  "
        );
        assert_eq!(&source[blocks[1].clone()], ".badge { color: blue; }");
    }

    #[test]
    fn transforms_only_embedded_style_contents() {
        let source = r#"@php($title = 'demo')
<div class="card">
  <style>
    .button { color: red; }
    .button:hover { color: blue; }
  </style>
  {{ $title }}
</div>
"#;
        let output = apply_until_stable(
            Path::new("view.blade.php"),
            source,
            &[RuleId::NestPseudoClass],
            false,
        )
        .unwrap();
        assert!(output.starts_with("@php($title = 'demo')\n<div class=\"card\">\n  <style>"));
        assert!(
            output.contains("&:hover"),
            "embedded CSS was not transformed: {output}"
        );
        assert!(output.contains("{{ $title }}"));
        assert!(output.ends_with("</div>\n"));
    }

    #[test]
    fn plans_selector_lists_inside_nested_style_rules() {
        let source = r#"<div>
  <style>
    .daterangepicker {
      .next span,
      .prev span {
        border-color: #64748b;
      }
    }
  </style>
</div>
"#;
        let report = analyze_content(
            PathBuf::from("view.blade.php"),
            source,
            &[RuleId::ModernizeIs],
        )
        .unwrap();
        assert_eq!(report.plans.len(), 1);
        assert_eq!(
            &source[report.plans[0].source_range.start..report.plans[0].source_range.end],
            ".next span,\n      .prev span "
        );

        let output = apply_until_stable(
            Path::new("view.blade.php"),
            source,
            &[RuleId::ModernizeIs],
            false,
        )
        .unwrap();
        assert!(output.contains(":is(.next, .prev) span"));
        assert!(output.contains("<div>"));
        assert!(output.contains("</style>"));
    }

    #[test]
    fn plans_rules_inside_nested_starting_style_at_rules() {
        let source = r#".dialog {
  @starting-style {
    .dialog-panel:hover,
    .dialog-panel:focus {
      opacity: 0;
    }
  }
}
"#;
        let report = analyze_content(
            PathBuf::from("nested-starting-style.css"),
            source,
            &[RuleId::ModernizeIs],
        )
        .unwrap();
        assert_eq!(report.plans.len(), 1);
        assert_eq!(report.plans[0].rules, vec![RuleId::ModernizeIs]);

        let output = apply_until_stable(
            Path::new("nested-starting-style.css"),
            source,
            &[RuleId::ModernizeIs],
            false,
        )
        .unwrap();
        assert!(output.contains(".dialog-panel:is(:hover, :focus)"));
        assert!(output.contains("@starting-style"));
    }

    #[test]
    fn preserves_nested_starting_style_when_processing_conditional_styles() {
        let source = r#"@supports selector(details::details-content) {
  .faq-list {
    details {
      &[open]::details-content {
        height: auto;
      }
    }

    @starting-style {
      details[open]::details-content {
        height: 0;
      }
    }
  }
}
"#;
        let output = apply_until_stable(
            Path::new("nested-starting-style.css"),
            source,
            &RuleId::ALL,
            true,
        )
        .unwrap();
        assert_eq!(output.matches("@starting-style").count(), 1, "{output}");
        assert!(
            output.contains("details[open]::details-content") && output.contains("height: 0;"),
            "nested starting-style declaration was lost or changed:\n{output}"
        );
    }

    #[test]
    fn preserves_utf8_declaration_values_during_nesting() {
        let source = "input[type=checkbox] { content: '✓'; }\n";
        let output = apply_until_stable(
            Path::new("utf8.css"),
            source,
            &[RuleId::NestAttribute],
            false,
        )
        .unwrap();
        assert!(
            output.contains("content: '✓';"),
            "UTF-8 value changed: {output}"
        );
        assert!(!output.contains("Ã"), "mojibake introduced: {output}");
    }

    #[test]
    fn does_not_treat_relative_or_already_nested_selectors_as_state_bases() {
        assert_eq!(extract_base_target("> [class*='col-']"), None);
        assert_eq!(extract_base_target(":is(.prev, .next):hover"), None);
        assert_eq!(extract_base_target("&:hover"), None);
        assert_eq!(extract_base_target(".button:hover"), Some(".button"));
    }

    #[test]
    fn does_not_reparent_existing_nesting_selectors() {
        assert_eq!(
            selector_relation(".card-body", "&.compact .card-body"),
            None
        );
        assert_eq!(selector_relation("select", "&:hover"), None);
    }

    #[test]
    fn marks_mixed_specificity_identical_body_merges_for_review() {
        let css =
            ".form-select option { color: white; }\nselect.form-control option { color: white; }\n";
        let plans = plan(css, &[RuleId::MergeIdenticalRuleBodies]);
        assert_eq!(plans.len(), 1);
        assert_eq!(plans[0].safety, Safety::Review);
        assert!(!plans[0].proof.specificity_equivalent);
    }

    #[test]
    fn already_nested_selector_lists_reach_a_fixed_point() {
        let css = ".card {
  &,
  &.active {
    color: red;
  }
}
";
        let output = apply_until_stable(Path::new("test.css"), css, &RuleId::ALL, false).unwrap();
        assert_eq!(output, css);
    }

    #[test]
    fn nests_adjacent_pseudo_and_descendant_rules() {
        let css = ".card {\n  color: red;\n}\n.card:hover {\n  color: blue !important;\n}\n.card .title {\n  font-weight: 700;\n}\n";
        let plans = plan(css, &RuleId::ALL);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("&:hover"));
        assert!(output.contains(".title"));
        assert!(output.contains("color: blue !important;"));
    }

    #[test]
    fn calculates_functional_pseudo_specificity_from_the_highest_argument() {
        assert_eq!(
            calculate_specificity(":is(.a, #b, div)"),
            Specificity {
                ids: 1,
                classes: 0,
                elements: 0,
            }
        );
        assert_eq!(
            calculate_specificity(":has(> .a, #b)"),
            Specificity {
                ids: 1,
                classes: 0,
                elements: 0,
            }
        );
        assert_eq!(
            calculate_specificity(":not(.a, #b)"),
            Specificity {
                ids: 1,
                classes: 0,
                elements: 0,
            }
        );
        assert_eq!(
            calculate_specificity(":where(#header, .nav, div)"),
            Specificity::default()
        );
        assert_eq!(
            calculate_specificity(".parent > :has(.a, #b)"),
            Specificity {
                ids: 1,
                classes: 1,
                elements: 0,
            }
        );
    }

    #[test]
    fn preserves_not_matching_logic_when_consolidating_only_chained_negations() {
        assert_eq!(
            consolidate_not_in_selector(".card:not(.a):not(.b)"),
            Some((".card:not(.a, .b)".to_string(), true))
        );
        assert_eq!(consolidate_not_in_selector(".card:not(.a, .b)"), None);
        assert_eq!(
            consolidate_not_in_selector(".card:not(.a), .card:not(.b)"),
            None
        );
    }

    #[test]
    fn keeps_has_arguments_as_relative_selectors_without_rewriting_the_logic() {
        let selector = ".parent:has(> .a, + #b, .c .d)";
        assert_eq!(
            calculate_specificity(selector),
            Specificity {
                ids: 1,
                classes: 1,
                elements: 0,
            }
        );
        assert_eq!(factor_with_is(selector), None);
        assert_eq!(factor_with_where(selector), None);
    }

    #[test]
    fn applies_overlapping_plans_until_the_source_is_stable() {
        let css = r#".states {
  border: 1px solid transparent;
}

.states > .bolt:is(:hover, :focus-visible) {
  outline: 2px solid currentColor;
}

.states > .bolt,
.states > .spark {
  min-block-size: 2rem;
}
"#;
        let output = apply_until_stable(Path::new("test.css"), css, &RuleId::ALL, false).unwrap();

        assert!(output.contains("& > .bolt:is(:hover, :focus-visible) {"));
        assert!(output.contains("& > :is(.bolt, .spark) {"));
        assert!(!output.contains(".states > .bolt,\n.states > .spark"));
    }

    #[test]
    fn nests_exact_full_modernize_example() {
        let original = r#".card {
  color: #222;
  padding: 1rem;
}
.card:hover {
  color: #111 !important;
}
.card::before {
  content: "";
}
.card[data-active] {
  border-color: currentColor;
}
.card.featured {
  box-shadow: 0 0 0 1px currentColor;
}
.card .title {
  font-weight: 700;
}
.card > .body {
  min-width: 0;
}
.card + .card {
  margin-top: 1rem;
}
@media (width >= 48rem) {
  .card {
    padding: 1.5rem;
  }
}
@supports (display: grid) {
  .card {
    display: grid;
  }
}
"#;

        let expected = r#".card {
  color: #222;
  padding: 1rem;

  &:hover {
    color: #111 !important;
  }

  &::before {
    content: "";
  }

  &[data-active] {
    border-color: currentColor;
  }

  &.featured {
    box-shadow: 0 0 0 1px currentColor;
  }

  .title {
    font-weight: 700;
  }

  & > .body {
    min-width: 0;
  }

  & + .card {
    margin-top: 1rem;
  }

  @media (width >= 48rem) {
    padding: 1.5rem;
  }

  @supports (display: grid) {
    display: grid;
  }
}"#;

        let plans = plan(
            original,
            &[
                RuleId::NestPseudoClass,
                RuleId::NestPseudoElement,
                RuleId::NestAttribute,
                RuleId::NestCompound,
                RuleId::NestDescendant,
                RuleId::NestCombinator,
                RuleId::NestMedia,
                RuleId::NestSupports,
            ],
        );
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(original, &plans, false).unwrap();
        assert_eq!(output.trim(), expected.trim());
    }

    #[test]
    fn factors_selector_list_sharing_base() {
        let css = ".marker,\n.marker::before,\n.marker::after {\n  box-sizing: border-box;\n}\n";
        let plans = plan(css, &[RuleId::FactorSelectorList]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".marker {"));
        assert!(output.contains("&,"));
        assert!(output.contains("&::before,"));
        assert!(output.contains("&::after {"));
        assert!(output.contains("box-sizing: border-box;"));
        assert!(output.contains("    box-sizing: border-box;"));
    }

    #[test]
    fn modernizes_is_with_uniform_specificity() {
        let css = ".button:hover, .button:focus, .button:active {\n  color: blue;\n}\n";
        let plans = plan(css, &[RuleId::ModernizeIs]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".button:is(:hover, :focus, :active)"));
    }

    #[test]
    fn never_factors_pseudo_elements_into_functional_pseudo_classes() {
        for selector in [
            ".icon:before, .icon:after",
            ".icon::before, .icon::after",
            ".icon:first-line, .icon:first-letter",
            ".icon:is(:before), .icon:is(:after)",
        ] {
            assert_eq!(
                factor_with_is(selector),
                None,
                "pseudo-element selector was incorrectly factored: {selector}"
            );
        }
    }

    #[test]
    fn does_not_factor_inside_functional_pseudo_arguments() {
        let selector = ".equivalent-has:has(> .alpha), .equivalent-has:has(> .beta)";
        assert_eq!(
            factor_with_is(selector),
            Some((".equivalent-has:has(> .alpha, > .beta)".to_string(), true))
        );

        let css = format!("{selector} {{\n  color: seagreen;\n}}\n");
        let plans = plan(&css, &[RuleId::ModernizeIs]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(&css, &plans, false).unwrap();
        assert!(output.contains(".equivalent-has:has(> .alpha, > .beta)"));
    }

    #[test]
    fn combines_uniform_is_and_where_function_arguments() {
        assert_eq!(
            factor_with_is(".card:is(.alpha), .card:is(.beta)"),
            Some((".card:is(.alpha, .beta)".to_string(), true))
        );
        assert_eq!(
            factor_with_is(".card:where(#alpha), .card:where(.beta)"),
            Some((".card:where(#alpha, .beta)".to_string(), true))
        );
    }

    #[test]
    fn refuses_to_combine_separate_not_functions() {
        assert_eq!(factor_with_is(".card:not(.alpha), .card:not(.beta)"), None);
    }

    #[test]
    fn modernizes_media_range_syntax() {
        let css = "@media (min-width: 800px) {\n  .card { padding: 2rem; }\n}\n";
        let plans = plan(css, &[RuleId::ModernizeMediaRange]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("@media (width >= 800px)"));
    }

    #[test]
    fn consolidates_not_selectors() {
        let css = "input:not([type=\"checkbox\"]):not([type=\"radio\"]) {\n  border: 1px solid gray;\n}\n";
        let plans = plan(css, &[RuleId::ConsolidateNot]);
        assert_eq!(plans.len(), 1);
        assert_eq!(plans[0].safety, Safety::Review);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains("input:not([type=\"checkbox\"], [type=\"radio\"])"));
    }

    #[test]
    fn refuses_subtoken_is_factoring_false_positive() {
        let css = ".same-specificity-a,\n.same-specificity-b {\n  color: black;\n}\n";
        let plans = plan(css, &[RuleId::ModernizeIs]);
        assert!(plans.is_empty());
    }

    #[test]
    fn modernizes_descendant_is_alternatives() {
        let css = ".card .title, .card .subtitle, .card .description {\n  color: black;\n}\n";
        let plans = plan(css, &[RuleId::ModernizeIs]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".card :is(.title, .subtitle, .description)"));
    }

    #[test]
    fn modernizes_suffix_is_alternatives() {
        let css = ".alpha .title,\n#hero .title {\n  color: rebeccapurple;\n}\n";
        let plans = plan(css, &[RuleId::ModernizeIs]);
        assert_eq!(plans.len(), 1);
        assert_eq!(plans[0].safety, Safety::Review);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains(":is(.alpha, #hero) .title"));
    }

    #[test]
    fn factors_multi_selector_cluster_with_is_and_nesting() {
        let css = r#".alpha .title,
#hero .title {
  color: rebeccapurple;
}

.alpha .subtitle,
#hero .subtitle {
  color: slateblue;
}

.alpha,
#hero {
  border-color: currentColor;
}
"#;
        let plans = plan(css, &[RuleId::ModernizeIs]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains(":is(.alpha, #hero) {"));
        assert!(output.contains("border-color: currentColor;"));
        assert!(output.contains(".title {"));
        assert!(output.contains("color: rebeccapurple;"));
        assert!(output.contains(".subtitle {"));
        assert!(output.contains("color: slateblue;"));
    }

    #[test]
    fn refuses_bem_token_concatenation() {
        let css = ".card { color: red; }\n.card__title { font-weight: 700; }\n";
        let plans = plan(css, &RuleId::ALL);
        assert!(plans.is_empty());
    }

    #[test]
    fn merges_same_named_layer_blocks() {
        let css = "@layer overrides {\n  .layered-card {\n    color: darkgreen;\n  }\n}\n\n@layer overrides {\n  .layer-important {\n    color: orange !important;\n  }\n}\n";
        let plans = plan(css, &[RuleId::MergeSameNamedLayer]);
        assert_eq!(plans.len(), 2);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("@layer overrides {"));
        assert!(output.contains(".layered-card {"));
        assert!(output.contains(".layer-important {"));
    }

    #[test]
    fn merges_adjacent_media_queries() {
        let css = "@media (width >= 48rem) {\n  .card {\n    padding: 2rem;\n  }\n}\n\n@media (width >= 48rem) {\n  .panel {\n    padding: 2rem;\n  }\n}\n";
        let plans = plan(css, &[RuleId::MergeAdjacentMedia]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("@media (width >= 48rem) {"));
        assert!(output.contains(".card {"));
        assert!(output.contains(".panel {"));
    }

    #[test]
    fn merges_adjacent_supports_queries() {
        let css = "@supports (display: grid) {\n  .card {\n    display: grid;\n  }\n}\n\n@supports (display: grid) {\n  .panel {\n    display: grid;\n  }\n}\n";
        let plans = plan(css, &[RuleId::MergeAdjacentSupports]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("@supports (display: grid) {"));
        assert!(output.contains(".card {"));
        assert!(output.contains(".panel {"));
    }

    #[test]
    fn merges_adjacent_identical_selectors() {
        let css = ".card {\n  color: black;\n}\n\n.card {\n  padding: 1rem;\n}\n";
        let plans = plan(css, &[RuleId::MergeAdjacentIdenticalSelector]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".card {"));
        assert!(output.contains("color: black;"));
        assert!(output.contains("padding: 1rem;"));
    }

    #[test]
    fn merges_identical_rule_bodies() {
        let css = ".card:hover {\n  color: red;\n}\n\n.panel:hover {\n  color: red;\n}\n";
        let plans = plan(css, &[RuleId::MergeIdenticalRuleBodies]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".card:hover,"));
        assert!(output.contains(".panel:hover {"));
        assert!(output.contains("color: red;"));
    }

    #[test]
    fn factors_identical_states_with_is() {
        let css = ".card:hover {\n  background: silver;\n}\n\n.card:focus {\n  background: silver;\n}\n\n.card:focus-visible {\n  background: silver;\n}\n";
        let plans = plan(css, &[RuleId::FactorIdenticalStatesWithIs]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".card {"));
        assert!(output.contains("&:is(:hover, :focus, :focus-visible) {"));
        assert!(output.contains("background: silver;"));
    }

    #[test]
    fn nests_multi_level_tree_hierarchy() {
        let css = r#".tree {
  display: grid;
  gap: 0.5rem;
}
.tree .node {
  position: relative;
}
.tree .node .label {
  display: flex;
}
.tree .node .label:hover {
  color: var(--accent);
}
.tree .node > .children {
  margin-inline-start: 1.25rem;
}
.tree .node > .children > .node + .node {
  margin-block-start: 0.25rem;
}
"#;
        let plans = plan(
            css,
            &[
                RuleId::NestDescendant,
                RuleId::NestCombinator,
                RuleId::NestPseudoClass,
            ],
        );
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".node {"));
        assert!(output.contains(".label {"));
        assert!(output.contains("&:hover {"));
        assert!(output.contains("> .children {"));
        assert!(output.contains("> .node + .node {"));
    }

    #[test]
    fn emits_explicit_nesting_selector_for_child_combinator() {
        let css = r#".c-command-hero {
  display: grid;
}

.c-command-hero > .c-bolt {
  z-index: 4;
}

.c-command-hero + .c-command-hero {
  margin-block-start: 1rem;
}

.c-command-hero ~ .c-command-note {
  color: gray;
}

.c-command-hero > .c-bolt > .c-bolt__icon {
  inline-size: 1rem;
}
"#;
        let plans = plan(css, &[RuleId::NestCombinator, RuleId::NestDescendant]);
        let output = apply_selected_plans(css, &plans, false).unwrap();

        assert!(output.contains("& > .c-bolt {"), "{output}");
        assert!(output.contains("& + .c-command-hero {"), "{output}");
        assert!(output.contains("& ~ .c-command-note {"), "{output}");
        assert!(
            output.contains("& > .c-bolt > .c-bolt__icon {"),
            "multi-level combinator should preserve its full selector: {output}"
        );
        assert!(
            !output.contains("& {\n"),
            "combinator must not be split into a nested block: {output}"
        );
        assert!(!output.contains("\n    > {"), "{output}");
        assert!(!output.contains("\n    + {"), "{output}");
        assert!(!output.contains("\n    ~ {"), "{output}");
    }

    #[test]
    fn nests_in_place_input_states() {
        let css = "input:user-invalid {\n  border-color: crimson;\n}\ninput:user-valid {\n  border-color: seagreen;\n}\ninput:placeholder-shown {\n  color: gray;\n}\n";
        let plans = plan(css, &[RuleId::NestPseudoClass]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains("input {"));
        assert!(output.contains("&:user-invalid {"));
        assert!(output.contains("&:user-valid {"));
        assert!(output.contains("&:placeholder-shown {"));
    }

    #[test]
    fn gathers_consecutive_conditions_by_selector() {
        let css = "@media (width >= 30rem) {\n  .responsive-grid {\n    gap: 1rem;\n  }\n}\n\n@media (width >= 80rem) {\n  .responsive-grid {\n    gap: 2rem;\n  }\n}\n";
        let plans = plan(css, &[RuleId::NestMedia]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".responsive-grid {"));
        assert!(output.contains("@media (width >= 30rem) {"));
        assert!(output.contains("@media (width >= 80rem) {"));
    }

    #[test]
    fn factors_selector_list_with_adjacent_hover() {
        let css = ".notice,\n.notice::before,\n.notice::after {\n  color: currentColor;\n}\n\n.notice:hover {\n  background: color-mix(in srgb, currentColor 8%, transparent);\n}\n";
        let plans = plan(css, &[RuleId::FactorSelectorList, RuleId::NestPseudoClass]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(".notice {"));
        assert!(output.contains("&,"));
        assert!(output.contains("&::before,"));
        assert!(output.contains("&::after {"));
        assert!(output.contains("&:hover {"));
        assert!(output.contains("background: color-mix"));
    }

    #[test]
    fn gathers_non_adjacent_related_selector_rules_with_nested_blocks() {
        let css = r#".skip-link {
    position: absolute;
    inset-block-start: -48px;
    inset-inline-start: 1rem;
    z-index: 10000000000;
    background: var(--bg-color);
    color: var(--text-color);
    border: 1px solid var(--border-color);
    border-radius: 0.5rem;
    padding: 0.55rem 0.8rem;
    text-decoration: none;
    font-weight: 700;
    transition: inset-block-start 0.2s ease;

    &:focus-visible {
        inset-block-start: 0.75rem;
    }
}

.unrelated-rule {
    color: red;
}

.skip-link {
    font: optional;

    &::after {
        content: '';
    }

    :not(*) & {
        all: unset
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains("font: optional;"));
    }

    #[test]
    fn gathers_non_adjacent_related_pseudo_and_combinator_rules() {
        let css = r#".skip-link {
    position: absolute;
    inset-block-start: -48px;
    inset-inline-start: 1rem;
    z-index: 10000000000;
    background: var(--bg-color);
    color: var(--text-color);
    border: 1px solid var(--border-color);
    border-radius: 0.5rem;
    padding: 0.55rem 0.8rem;
    text-decoration: none;
    font-weight: 700;
    transition: inset-block-start 0.2s ease;

    &:focus-visible {
        inset-block-start: 0.75rem;
    }
}

.unrelated {
    color: red;
}

.skip-link {
    font: optional;

    &::after {
        content: '';
    }

    :not(*) & {
        all: unset
    }
}

.skip-link+* {
    display: block;
}

.skip-link::backdrop {
    background-color: gray;
}

.skip-link:has(*) {
    color: #27ca3f;
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains("font: optional;"));
        assert!(output.contains("+ * {"));
        assert!(output.contains("&::backdrop {"));
        assert!(output.contains("&:has(*) {"));
    }

    #[test]
    fn gather_keeps_explicit_combinators_as_complete_nested_selectors() {
        let css = r#".c-command-hero {
  display: grid;
}

.c-command-hero > .c-bolt {
  z-index: 4;
}

.unrelated {
  color: gray;
}

.c-command-hero {
  border-radius: 0.75rem;
}

.c-command-hero + .c-command-hero {
  margin-block-start: 1rem;
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();

        assert!(output.contains("& > .c-bolt {"), "{output}");
        assert!(
            !output.contains("& {\n"),
            "explicit combinator must not be split: {output}"
        );
        assert!(!output.contains("\n    > .c-bolt {"), "{output}");
    }

    #[test]
    fn test_modernizes_media_range_syntax_no_whitespace() {
        let input = "@media (max-width:60rem) { .content { width: 100%; } }";
        let plans = plan(input, &[RuleId::ModernizeMediaRange]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(input, &plans, true).unwrap();
        assert!(output.contains("(width <= 60rem)"));
    }

    #[test]
    fn plans_continue_when_lightningcss_rejects_picker_pseudo() {
        // `::picker()` is valid CSS but LightningCSS rejects it. Planning must
        // still run from the structural scanner so siblings can nest/modernize.
        let css = r#".custom-select {
    appearance: none;
}

.custom-select:hover {
    border-color: teal;
}

.custom-select::picker(select) {
    background: white;
}

.custom-select::picker(select)::-webkit-scrollbar {
    width: 6px;
}

@media (max-width: 768px) {
    .custom-select {
        inline-size: 100%;
    }
}
"#;
        let report = analyze_source(PathBuf::from("picker.css"), css, &RuleId::ALL).unwrap();
        eprintln!(
            "parse_ok={} err={:?} plans={}",
            report.parse_ok,
            report.parse_error,
            report.plans.len()
        );
        for p in &report.plans {
            eprintln!("  {:?} {:?}", p.rules, p.reason);
        }
        assert!(
            !report.plans.is_empty(),
            "must still emit plans (parse_ok={:?} err={:?}): findings={:?}",
            report.parse_ok,
            report.parse_error,
            report.findings
        );
        let output = apply_selected_plans(css, &report.plans, true).unwrap();
        assert!(
            output.contains("&:hover") || output.contains("&::picker"),
            "custom-select relatives should nest: {output}"
        );
        assert!(
            output.contains("(width <= 768px)") || output.contains("inline-size: 100%"),
            "media-range or nest should still apply: {output}"
        );
    }

    #[test]
    fn nesting_adds_semicolon_to_last_declaration_without_semicolon() {
        // .tabpanel body ends with `display: block !important` (no `;`)
        // After nesting the combinator, the declaration must get a `;` inserted
        // so it doesn't run together with the opening `{` of the nested rule.
        let css = r#".tabpanel {
  display: block !important
}

.tabpanel+.tabpanel {
  margin-block-start: .5rem
}
"#;
        let rules = &[
            RuleId::NestCombinator,
            RuleId::NestDescendant,
            RuleId::NestPseudoClass,
            RuleId::NestPseudoElement,
        ];
        let plans = plan(css, rules);
        assert!(!plans.is_empty(), "expected at least one nesting plan");
        let output = apply_selected_plans(css, &plans, true).unwrap();
        // Must NOT contain the malformed concatenation
        assert!(
            !output.contains("!important+"),
            "semicolon missing before nested rule: {output}"
        );
        // Must contain properly terminated declaration
        assert!(
            output.contains("!important;") || output.contains("!important\n"),
            "declaration should end with ';': {output}"
        );
        // Nested rule selector must appear on its own
        assert!(
            output.contains("+ .tabpanel {") || output.contains("+.tabpanel {"),
            "nested combinator rule missing: {output}"
        );
    }

    #[test]
    fn preserves_multiline_custom_property_values_when_nesting() {
        let css = r#".card {
  --shadow:
    0 1px 2px rgb(0 0 0 / 8%),
    0 4px 12px rgb(0 0 0 / 5%);
}

.card:hover {
  color: blue;
}
"#;
        let plans = plan(css, &[RuleId::NestPseudoClass]);
        assert!(!plans.is_empty(), "expected a pseudo-class nesting plan");
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("--shadow:\n"),
            "custom property was altered: {output}"
        );
        assert!(
            output.contains("0 1px 2px rgb(0 0 0 / 8%),\n    0 4px 12px rgb(0 0 0 / 5%);")
                || output.contains("0 1px 2px rgb(0 0 0 / 8%),\n      0 4px 12px rgb(0 0 0 / 5%);"),
            "custom property value was reformatted: {output}"
        );
        assert!(
            !output.contains("--shadow:;"),
            "multiline value was terminated early: {output}"
        );
    }

    #[test]
    fn gather_related_selector_rules_adds_semicolon_to_declarations_without_semicolon() {
        // Declarations without trailing `;` must get one inserted when gathered
        // into the canonical block so they don't corrupt nested rule opening braces.
        let css = r#".skip-link {
    position: absolute;
    font-weight: 700
}

.unrelated { color: red }

.skip-link:focus {
    outline: 2px solid currentColor
}

.skip-link+* {
    display: block
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert!(!plans.is_empty(), "expected gather plan");
        let output = apply_selected_plans(css, &plans, true).unwrap();
        // No declaration should run together with `{`
        assert!(
            !output.contains("700\n    &") && !output.contains("700&") && !output.contains("700{"),
            "semicolon missing before nested pseudo/combinator: {output}"
        );
        assert!(
            !output.contains("currentColor\n    + *") && !output.contains("currentColor{"),
            "semicolon missing before nested combinator: {output}"
        );
        // All gathered declarations must end with `;`
        assert!(
            output.contains("font-weight: 700;"),
            "missing ';' after font-weight: {output}"
        );
        assert!(
            output.contains("position: absolute;"),
            "missing ';' after position: {output}"
        );
    }

    #[test]
    fn gathers_non_adjacent_related_rule_inside_media_query() {
        let css = r#".skip-link {
    position: absolute;
    font-weight: 700;

    &:focus-visible {
        inset-block-start: 0.75rem;
    }
}

.unrelated {
    color: red;
}

@media (width <= 1024px) {
    .skip-link :not(*) {
        position: inherit
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert!(
            !plans.is_empty(),
            "expected gather plan for media-wrapped related rule"
        );
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            !output.contains(".skip-link :not(*)"),
            "flat media descendant should be gathered: {output}"
        );
        assert!(
            output.contains(":not(*) {"),
            "descendant :not(*) should nest under .skip-link: {output}"
        );
        assert!(
            output.contains("@media (width <= 1024px) {"),
            "media query should nest inside :not(*): {output}"
        );
        assert!(
            output.contains("position: inherit"),
            "declaration should be preserved: {output}"
        );
        let not_pos = output.find(":not(*) {").expect(":not(*)");
        let media_pos = output.find("@media (width <= 1024px) {").expect("@media");
        assert!(
            media_pos > not_pos,
            "media must nest inside :not(*), not the reverse: {output}"
        );
    }

    #[test]
    fn gathers_exact_parent_inside_media_as_nested_at_rule() {
        let css = r#".skip-link {
    position: absolute;
}

.unrelated { color: red }

@media (width <= 600px) {
    .skip-link {
        display: none
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains("@media (width <= 600px) {"), "{output}");
        assert!(output.contains("display: none;"), "{output}");
        assert!(
            !output.contains("@media (width <= 600px) {\n    .skip-link"),
            "should invert to .skip-link {{ @media }}: {output}"
        );
    }

    #[test]
    fn gather_media_leaves_unrelated_siblings_in_place() {
        let css = r#".skip-link {
    position: absolute;
}

.unrelated { color: red }

@media (width <= 1024px) {
    .skip-link :not(*) {
        position: inherit
    }
    .other {
        color: blue
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains(":not(*) {"), "{output}");
        assert!(
            output.contains(".other") && output.contains("color: blue"),
            "unrelated sibling must stay in the media block: {output}"
        );
    }

    #[test]
    fn nests_appended_nesting_selector_descendant() {
        // MDN: .featured .card → .card { .featured & { ... } }
        let css = ".card {\n  color: red;\n}\n.featured .card {\n  border: 1px solid;\n}\n";
        let plans = plan(css, &[RuleId::NestDescendant]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(
            output.contains(".featured & {"),
            "expected appended &: {output}"
        );
        assert!(output.contains("border: 1px solid;"), "{output}");
    }

    #[test]
    fn nests_appended_nesting_selector_not() {
        // MDN-adjacent: :not(.card) → .card { :not(&) { ... } }
        let css = ".card {\n  color: red;\n}\n:not(.card) {\n  display: none;\n}\n";
        let plans = plan(css, &[RuleId::NestPseudoClass]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(output.contains(":not(&) {"), "expected :not(&): {output}");
        assert!(output.contains("display: none;"), "{output}");
    }

    #[test]
    fn nests_appended_compound_selector() {
        let css = ".card {\n  color: red;\n}\n.featured.card {\n  font-weight: 700;\n}\n";
        let plans = plan(css, &[RuleId::NestCompound]);
        assert_eq!(plans.len(), 1);
        let output = apply_selected_plans(css, &plans, false).unwrap();
        assert!(
            output.contains(".featured& {"),
            "expected compound appended &: {output}"
        );
    }

    #[test]
    fn gathers_non_adjacent_appended_nesting_selector() {
        let css = r#".card {
    color: red;
}

.unrelated { color: blue }

.featured .card {
    border: 1px solid
}

:not(.card) {
    display: none
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert!(!plans.is_empty());
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains(".featured & {"), "{output}");
        assert!(output.contains(":not(&) {"), "{output}");
        assert!(!output.contains(".featured .card"), "{output}");
        assert!(!output.contains(":not(.card)"), "{output}");
    }

    #[test]
    fn gather_does_not_nest_selector_list_under_one_branch() {
        // `A, B { shared }` must not become `B { A, & { shared } }` —
        // that turns A into a descendant of B.
        let css = r#"::view-transition-old(root),
::view-transition-new(root) {
    position: absolute;
    inset: 0;
    animation: 0.55s ease-in-out both;
}

.unrelated { color: red }

::view-transition-old(root) {
    animation-name: fadeOut;
}

::view-transition-new(root) {
    animation-name: fadeIn;
}
"#;
        let plans = plan(
            css,
            &[
                RuleId::GatherRelatedSelectorRules,
                RuleId::FactorSelectorList,
            ],
        );
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            !output.contains("::view-transition-old(root),\n    &")
                && !output.contains("::view-transition-old(root),\n    & {")
                && !output.contains("::view-transition-old(root),\n        &"),
            "selector list must not nest under one branch: {output}"
        );
        assert!(
            output.contains("::view-transition-old(root)")
                && output.contains("::view-transition-new(root)")
                && output.contains("animation-name: fadeOut")
                && output.contains("animation-name: fadeIn"),
            "both branches and their unique decls must remain: {output}"
        );
        assert!(
            output.contains("position: absolute"),
            "shared declarations must be kept: {output}"
        );
    }

    #[test]
    fn gather_nests_not_focus_visible_as_non_relative_amp() {
        // `&` anywhere (here inside `:not()`) makes the nest non-relative:
        // `:focus-visible { .skip-link:focus:not(&) }` ≡
        // `.skip-link:focus:not(:is(:focus-visible))` — no descendant combinator.
        assert!(selector_contains_nesting_amp(".skip-link:focus:not(&)"));
        assert!(!selector_contains_nesting_amp(".skip-link:focus"));

        let css = r#":focus-visible {
    outline: 2px solid blue;
}

.unrelated { color: red }

/* Preserve visible focus for skip-link activation (any modality). */
.skip-link:focus:not(:focus-visible) {
    outline: 2px solid blue;
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains(".skip-link:focus:not(&)"),
            "should nest as non-relative :not(&): {output}"
        );
        assert!(
            !output.contains(":focus-visible .skip-link")
                && !output.contains(":focus-visible  .skip-link"),
            "must not insert a descendant combinator: {output}"
        );
        assert!(
            output.contains("Preserve visible focus for skip-link activation"),
            "leading comment must move with the gathered rule: {output}"
        );
        let cmt = output.find("Preserve visible focus").expect("comment");
        let nest = output.find(".skip-link:focus:not(&)").expect("nest");
        assert!(
            cmt < nest,
            "comment should precede the nested rule: {output}"
        );
    }

    #[test]
    fn gather_nests_skip_link_focus_under_skip_link_not_focus_visible() {
        let css = r#":focus-visible {
    outline: 2px solid blue;
}

.skip-link {
    position: fixed;
}

.unrelated { color: red }

.skip-link:focus:not(:focus-visible) {
    outline: 2px solid blue;
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("&:focus:not(:focus-visible)"),
            "should nest under .skip-link: {output}"
        );
        assert!(
            !output.contains(".skip-link:focus:not(&)"),
            "must not nest under :focus-visible: {output}"
        );
    }

    #[test]
    fn gather_merges_same_nested_selector_from_style_and_media() {
        let css = r#".demo-out-text {
    font-size: 1.5rem;

    &.flash {
        animation: demo-out-flash .4s ease;
    }
}

.unrelated { color: red }

@media (prefers-reduced-motion: reduce) {
    .demo-out-text.flash {
        animation: none
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        let flash_opens = output.matches("&.flash {").count();
        assert_eq!(
            flash_opens, 1,
            "duplicate &.flash nests should merge: {output}"
        );
        assert!(output.contains("animation: demo-out-flash"), "{output}");
        assert!(
            output.contains("@media (prefers-reduced-motion: reduce)"),
            "{output}"
        );
        assert!(output.contains("animation: none"), "{output}");
    }

    #[test]
    fn gather_prefers_existing_specific_home_over_universal_append() {
        // `.skip-link + *` and `.skip-link:has(*)` can also be written as
        // `*` { `.skip-link+&` / `.skip-link:has(&)` }. The existing `.skip-link`
        // rule is the stronger home — do not duplicate into `*`.
        let css = r#".skip-link {
    position: absolute;
    font-weight: 700;
}

* {
    margin: 0;
}

.unrelated { color: red }

.skip-link+* {
    display: block
}

.skip-link:has(*) {
    color: #27ca3f
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("+ * {") || output.contains("+* {"),
            "combinator should nest under .skip-link: {output}"
        );
        assert!(
            output.contains("&:has(*) {"),
            ":has(*) should nest under .skip-link: {output}"
        );
        assert!(
            !output.contains(".skip-link+&")
                && !output.contains(".skip-link + &")
                && !output.contains(".skip-link:has(&)"),
            "must not append the same rules into *: {output}"
        );
        let star = output.find("\n* {").or_else(|| output.find("* {"));
        if let Some(star_at) = star {
            let after_star = &output[star_at..];
            let star_body = after_star.split('}').next().unwrap_or(after_star);
            assert!(
                !star_body.contains("skip-link"),
                "* must not absorb skip-link rules: {output}"
            );
        }
    }

    #[test]
    fn precise_conditional_home_is_not_hidden_by_universal_gather() {
        let css = r#"*
{
    margin: 0;
}

.faq-list {
    display: grid;
}

@supports selector(details::details-content) {
    .faq-list {
        details {
            &::details-content {
                height: 0;
            }
        }

        @starting-style {
            details[open]::details-content {
                height: 0;
            }
        }
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        assert!(
            plans.iter().any(|p| p.reason.contains("'.faq-list'")),
            "precise conditional home was suppressed: {plans:?}"
        );
        assert!(
            !plans.iter().any(|p| p.reason.contains("'*'")),
            "universal gather should not be planned: {plans:?}"
        );
    }

    #[test]
    fn gather_appends_only_when_no_prefix_home_exists() {
        let css = r#".card {
    color: red;
}

.unrelated { color: blue }

.featured .card {
    border: 1px solid
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(output.contains(".featured & {"), "{output}");
        assert!(!output.contains(".featured .card"), "{output}");
    }

    #[test]
    fn gather_keeps_supports_block_with_comma_list_and_mixed_homes() {
        let css = r#".custom-select {
    color: navy;
}

.unrelated { color: red }

@supports (appearance: base-select) {
    .custom-select,
    .custom-select::picker(select) {
        appearance: base-select;
    }
    .custom-select button { display: flex }
    selectedcontent { display: flex }
    .custom-select option { padding: 1rem }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("appearance: base-select"),
            "comma-list body must not be dropped: {output}"
        );
        assert!(
            output.contains("selectedcontent") && output.contains(".custom-select button")
                || output.contains("button {"),
            "mixed @supports inners must remain: {output}"
        );
    }

    #[test]
    fn gather_keeps_busy_mixed_media_grouped() {
        let css = r#".nav-links { display: flex; }
.hamburger-menu { display: none; }
.nav-controls { gap: 1rem; }

.unrelated { color: red }

@media (width <= 1024px) {
    .nav-links { display: none }
    .hamburger-menu { display: flex }
    .nav-controls { margin-inline-start: auto }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("@media (width <= 1024px)"),
            "mixed media with 3+ selectors should stay grouped: {output}"
        );
        assert!(
            !output.contains(".nav-links {\n    display: flex;\n\n    @media")
                && !output.contains(".hamburger-menu {\n    display: none;\n\n    @media"),
            "should not explode a busy media query into each parent: {output}"
        );
    }

    #[test]
    fn appended_nesting_does_not_match_ident_suffix() {
        let css = ".card {\n  color: red;\n}\n.mycard {\n  color: blue;\n}\n";
        let plans = plan(css, &[RuleId::NestCompound, RuleId::NestDescendant]);
        assert!(
            plans.is_empty(),
            ".mycard must not nest under .card: {:?}",
            plans.iter().map(|p| &p.proposed).collect::<Vec<_>>()
        );
    }

    #[test]
    fn gather_preserves_nested_selector_lists() {
        // `&::before,` contains `:` but is a selector-list continuation, not a declaration.
        let css = r#"* {
    margin: 0;
}

.unrelated { color: red }

@media (prefers-reduced-motion: reduce) {
    * {
        &,
        &::before,
        &::after {
            animation-duration: 0.001ms !important
        }
    }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            !output.contains("&::before,\n        ;")
                && !output.contains("&::before,;")
                && !output.contains("&,\n        ;")
                && !output.contains("&,;"),
            "selector-list comma must not become a declaration terminator: {output}"
        );
        assert!(
            output.contains("&::before,") && output.contains("&::after {"),
            "compound pseudo selector list must stay intact: {output}"
        );
        let before = output.find("&::before,").expect("before");
        let after = output.find("&::after {").expect("after");
        let between = &output[before..after];
        assert!(
            !between.contains(';'),
            "no semicolon between selector-list items: {between:?} in {output}"
        );
    }

    #[test]
    fn gather_keeps_every_custom_select_chrome_rule() {
        let css = r#".custom-select {
    color: navy;
}

@supports (appearance: base-select) {
    .custom-select,
    .custom-select::picker(select) {
        appearance: base-select;
    }
    .custom-select button {
        display: flex;
    }
    .custom-select button:hover {
        border-color: teal;
    }
    .custom-select:focus button,
    .custom-select button:focus-visible {
        outline: none;
    }
    .custom-select .select-arrow {
        color: gray;
    }
    .custom-select:open .select-arrow {
        rotate: -180deg;
    }
    .custom-select:open button {
        border-color: teal;
    }
    selectedcontent {
        display: flex;
    }
    selectedcontent .opt-icon {
        display: none;
    }
    .custom-select::picker(select) {
        background: white;
    }
    .custom-select:not(:open)::picker(select) {
        opacity: 0;
    }
    .custom-select option {
        padding: 1rem;
    }
    .custom-select .dropdown-search-container {
        position: sticky;
    }
    .custom-select .select-search {
        inline-size: 100%;
    }
    .custom-select option .opt-icon {
        color: gray;
    }
    .custom-select option:hover .opt-icon,
    .custom-select option:checked .opt-icon {
        color: teal;
    }
    .custom-select::picker-icon {
        display: none;
    }
}
"#;
        let rules: Vec<RuleId> = RuleId::ALL
            .iter()
            .copied()
            .filter(|r| *r != RuleId::ModernizeWhere)
            .collect();
        let plans = plan(css, &rules);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        for needle in [
            "appearance: base-select",
            "display: flex",
            "border-color: teal",
            "outline: none",
            "color: gray",
            "rotate: -180deg",
            "selectedcontent",
            "display: none",
            "background: white",
            "opacity: 0",
            "padding: 1rem",
            "position: sticky",
            "inline-size: 100%",
            "::picker-icon",
        ] {
            assert!(
                output.contains(needle),
                "lost `{needle}` after apply:\n{output}"
            );
        }
        assert!(
            output.contains("button") && output.contains("&:hover"),
            "button:hover should nest as button {{ &:hover }}:\n{output}"
        );
        assert!(
            !output.contains("&:open &") && !output.contains("&:focus &"),
            "must not rewrite :open/:focus descendants as appended &:\n{output}"
        );
        assert!(
            output.contains("&:open")
                && (output.contains("&:open {")
                    || output.contains("&:open .select-arrow")
                    || output.contains("&:open button")),
            ":open children should group under .custom-select:\n{output}"
        );
        assert!(
            output.contains("&:focus button") || output.contains("button:focus-visible"),
            "focus comma-list must stay as relative branches:\n{output}"
        );
        assert!(
            output.contains("selectedcontent")
                && (output.contains(".opt-icon") || output.contains("& .opt-icon")),
            "selectedcontent leftover must remain:\n{output}"
        );
        assert!(
            output.contains(".custom-select")
                && output.contains("@supports (appearance: base-select)"),
            "supports should invert under .custom-select:\n{output}"
        );
        let home = output.find(".custom-select {").expect("home");
        let supports = output
            .find("@supports (appearance: base-select)")
            .expect("supports");
        assert!(
            supports > home,
            "appearance supports should sit inside .custom-select:\n{output}"
        );
        assert!(
            !output.contains("@supports (appearance: base-select) {\n    .custom-select"),
            "must not leave a wrapper .custom-select inside @supports:\n{output}"
        );
    }

    #[test]
    fn gather_folds_unlayered_picker_into_layered_home() {
        let css = r#"@layer components {
    .custom-select {
        color: navy;
        &:is(:hover) { color: teal; }
    }
}

.custom-select::picker(select) {
    scrollbar-width: thin;
}
.custom-select::picker(select)::-webkit-scrollbar {
    width: 6px;
}
"#;
        let plans = plan(
            css,
            &[
                RuleId::GatherRelatedSelectorRules,
                RuleId::NestLayerBySelector,
            ],
        );
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("@layer components") && output.contains(".custom-select"),
            "layered home must stay in its layer:\n{output}"
        );
        assert!(
            output.contains(".custom-select::picker(select)")
                || output.contains("&::picker(select)"),
            "picker chrome must remain:\n{output}"
        );
        let layer_at = output.find("@layer components").expect("layer");
        let picker_flat = output.find(".custom-select::picker(select)");
        if let Some(picker_at) = picker_flat {
            assert!(
                picker_at < layer_at || !output[layer_at..].contains("scrollbar-width"),
                "unlayered picker must not be dumped into @layer components:\n{output}"
            );
        }
    }

    #[test]
    fn gather_does_not_dump_root_from_base_into_tokens() {
        let css = r#"@layer tokens {
    :root {
        color-scheme: light dark;
    }
}

@layer base {
    :root {
        scroll-behavior: smooth;
    }
    body { margin: 0; }
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("@layer tokens") && output.contains("@layer base"),
            "both layers must remain:\n{output}"
        );
        let tokens = output.split("@layer base").next().unwrap_or(&output);
        assert!(
            !tokens.contains("scroll-behavior"),
            "base :root decls must not move into tokens:\n{output}"
        );
        assert!(
            output.contains("scroll-behavior: smooth"),
            "base :root decls must survive:\n{output}"
        );
    }

    #[test]
    fn nest_layer_by_selector_preserves_layer_identity() {
        let css = r#"@layer tokens {
    :root {
        color-scheme: light dark;
    }
}

@layer base {
    :root {
        scroll-behavior: smooth;
    }
    body { margin: 0; }
}
"#;
        let plans = plan(css, &[RuleId::NestLayerBySelector]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains(":root {")
                && output.contains("@layer tokens {")
                && output.contains("@layer base {"),
            "should hoist :root and nest named layers:\n{output}"
        );
        assert!(
            output.contains("color-scheme: light dark")
                && output.contains("scroll-behavior: smooth")
                && output.contains("body"),
            "all declarations must survive:\n{output}"
        );
        // Must not create child layers like tokens.base
        assert!(
            !output.contains("@layer tokens {\n    :root")
                || output.find(":root {").unwrap() < output.find("@layer tokens {").unwrap_or(0)
                || output.matches("@layer tokens").count() >= 1,
            "{output}"
        );
        let root_at = output.find(":root {").expect("root");
        let tokens_inner = output[root_at..]
            .find("@layer tokens {")
            .expect("nested tokens");
        let base_inner = output[root_at..]
            .find("@layer base {")
            .expect("nested base");
        assert!(
            tokens_inner < base_inner,
            "layer order tokens then base must be preserved:\n{output}"
        );
        assert!(
            output.contains("body {") || output.contains("body{"),
            "unrelated base rules stay in @layer base:\n{output}"
        );
    }

    #[test]
    fn nest_layer_blocks_are_not_factored_into_anonymous_layer() {
        // Second-pass gather of two :root blocks must not turn
        // `@layer tokens { }` + `@layer base { }` into
        // `@layer { tokens {} base {} }`.
        let css = r#":root {
    @layer tokens {
        color-scheme: light dark;
    }

    @layer base {
        scroll-behavior: smooth;
    }
}

:root {
    scrollbar-width: thin;
}
"#;
        let plans = plan(
            css,
            &[
                RuleId::GatherRelatedSelectorRules,
                RuleId::NestLayerBySelector,
            ],
        );
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("@layer tokens") && output.contains("@layer base"),
            "named layers must stay named:\n{output}"
        );
        assert!(
            !output.contains("@layer {\n        tokens")
                && !output.contains("@layer {\n    tokens")
                && !output.contains("@layer {\n        base"),
            "must not wrap layer names as type selectors:\n{output}"
        );
        assert!(
            output.contains("color-scheme: light dark")
                && output.contains("scroll-behavior: smooth")
                && output.contains("scrollbar-width: thin"),
            "declarations must survive:\n{output}"
        );
    }

    #[test]
    fn nest_layer_skips_when_unlayered_rule_intervenes() {
        let css = r#"@layer base {
    .container { color: red; }
}

.some-rule { color: blue; }

@layer layout {
    .container { color: green; }
}
"#;
        let plans = plan(css, &[RuleId::NestLayerBySelector]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("@layer base")
                && output.contains("@layer layout")
                && output.contains(".some-rule"),
            "intervening unlayered rule blocks the hoist:\n{output}"
        );
        assert!(
            !output.contains(".container {\n    @layer base"),
            "must not move layout across .some-rule:\n{output}"
        );
    }

    #[test]
    fn gather_factors_compact_descendants_under_option_card() {
        let css = r#".option-card.compact .option-label {
    padding: 1rem;
}

.option-card.compact .option-icon {
    inline-size: 24px;
}

.unrelated { color: red }

.option-card {
    position: relative;
    cursor: pointer;
}
"#;
        let plans = plan(css, &[RuleId::GatherRelatedSelectorRules]);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("&.compact")
                && output.contains(".option-label")
                && output.contains(".option-icon"),
            "compact descendants should nest under .option-card:\n{output}"
        );
        assert!(
            output.contains("&.compact {")
                && output.contains(".option-label {")
                && output.contains(".option-icon {"),
            "shared &.compact prefix should be factored:\n{output}"
        );
        assert!(output.contains("position: relative"), "{output}");
        assert!(output.contains("padding: 1rem"), "{output}");
        assert!(output.contains("inline-size: 24px"), "{output}");
    }

    #[test]
    fn gather_does_not_orphan_deletes_when_shorter_nest_wins() {
        let css = r#".custom-select button {
    display: flex;
}
.custom-select button:hover {
    color: red;
}

.unrelated { color: blue }

.custom-select::picker(select) {
    background: white;
}
.custom-select::picker-icon {
    display: none;
}
"#;
        let rules = &[
            RuleId::NestPseudoClass,
            RuleId::NestDescendant,
            RuleId::GatherRelatedSelectorRules,
        ];
        let plans = plan(css, rules);
        let output = apply_selected_plans(css, &plans, true).unwrap();
        assert!(
            output.contains("background: white") && output.contains("display: none"),
            "picker chrome must survive mixed nest+gather:\n{output}"
        );
        assert!(
            output.contains("display: flex") && output.contains("color: red"),
            "button rules must survive:\n{output}"
        );
    }
}