text_typeset/

document_flow.rs

//! Per-widget document flow state.
//!
//! A [`DocumentFlow`] is everything that describes **what a specific
//! widget is showing** — viewport, zoom, scroll offset, wrap mode,
//! the laid-out flow (blocks / tables / frames), the rendered frame
//! cache, the cursor(s), and the selection / caret / text colors.
//!
//! Flows do not own font data. Every layout and render call takes a
//! [`TextFontService`] by reference and reads the font registry,
//! glyph atlas, and glyph cache through it. This split lets many
//! widgets in the same window share one atlas (and one GPU upload
//! per frame) while each owns an independent view onto its own
//! document.
//!
//! # Lifecycle
//!
//! ```rust,no_run
//! use text_typeset::{DocumentFlow, TextFontService};
//!
//! let mut service = TextFontService::new();
//! let face = service.register_font(include_bytes!("../test-fonts/NotoSans-Variable.ttf"));
//! service.set_default_font(face, 16.0);
//!
//! let mut flow = DocumentFlow::new();
//! flow.set_viewport(800.0, 600.0);
//!
//! # #[cfg(feature = "text-document")]
//! # {
//! let doc = text_document::TextDocument::new();
//! doc.set_plain_text("Hello, world!").unwrap();
//! flow.layout_full(&service, &doc.snapshot_flow());
//! # }
//!
//! let frame = flow.render(&mut service);
//! // frame.glyphs     -> glyph quads (textured rects from the shared atlas)
//! // frame.decorations -> cursor, selection, underlines, borders
//! ```
//!
//! The caller's pattern for a multi-widget UI is the same, plus one
//! rule: each widget owns its own `DocumentFlow` and must re-push
//! its view state (viewport, zoom, scroll, cursor, colors) before
//! its own `layout_*` / `render` call, because those fields live on
//! the flow itself, not on the shared service.

use crate::TextFontService;
use crate::font::resolve::resolve_font;
use crate::layout::block::BlockLayoutParams;
use crate::layout::flow::{FlowItem, FlowLayout};
use crate::layout::frame::FrameLayoutParams;
use crate::layout::inline_markup::{InlineAttrs, InlineMarkup};
use crate::layout::paragraph::{Alignment, break_into_lines};
use crate::layout::table::TableLayoutParams;
use crate::shaping::run::{ShapedGlyph, ShapedRun};
use crate::shaping::shaper::{bidi_runs, font_metrics_px, shape_text, shape_text_with_fallback};
use crate::types::{
    BlockVisualInfo, CharacterGeometry, CursorDisplay, DecorationKind, DecorationRect, GlyphQuad,
    HitTestResult, LaidOutSpan, LaidOutSpanKind, ParagraphResult, RenderFrame, SingleLineResult,
    TextFormat,
};

/// Reasons [`DocumentFlow::relayout_block`] may refuse an
/// incremental update.
///
/// Both variants describe invariant violations the caller can
/// detect structurally ahead of time by asking
/// [`DocumentFlow::has_layout`] and
/// [`DocumentFlow::layout_dirty_for_scale`]. Returned as a
/// `Result` rather than panicking so a misbehaving caller
/// produces a recoverable error at the exact call site instead
/// of corrupting the flow with a partial relayout.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum RelayoutError {
    /// No `layout_*` method has been called on this flow yet.
    /// The caller must run [`DocumentFlow::layout_full`] or
    /// [`DocumentFlow::layout_blocks`] first to establish a
    /// baseline layout before incremental updates make sense.
    NoLayout,
    /// The backing [`TextFontService`] has had its HiDPI scale
    /// factor changed since this flow was last laid out, so the
    /// existing block layouts hold advances at the old ppem.
    /// Re-shaping a single block would leave it at the new ppem
    /// while neighbors stay at the old, producing an inconsistent
    /// flow. The caller must re-run `layout_full` /
    /// `layout_blocks` to rebuild everything at the new scale.
    ScaleDirty,
}

impl std::fmt::Display for RelayoutError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            RelayoutError::NoLayout => {
                f.write_str("relayout_block called before any layout_* method")
            }
            RelayoutError::ScaleDirty => f.write_str(
                "relayout_block called after a scale-factor change without a fresh layout_*",
            ),
        }
    }
}

impl std::error::Error for RelayoutError {}

/// How the content (layout) width is determined.
///
/// Controls whether text reflows when the viewport resizes (web or
/// editor style) or wraps at a fixed width (page / WYSIWYG style).
#[derive(Debug, Clone, Copy, Default)]
pub enum ContentWidthMode {
    /// Content width equals viewport width (divided by zoom). Text
    /// reflows on window resize — the default, typical for editors
    /// and web layout.
    #[default]
    Auto,
    /// Content width is fixed at a specific value, independent of
    /// the viewport. Useful for page-like WYSIWYG layout, print
    /// preview, or side panels with their own column width.
    Fixed(f32),
}

/// Per-widget document flow state.
///
/// See the [module docs](self) for the shape of the split and for
/// lifecycle examples. Every layout/render method here takes a
/// [`TextFontService`] reference so flows can share one atlas across
/// an entire window.
pub struct DocumentFlow {
    flow_layout: FlowLayout,
    render_frame: RenderFrame,
    scroll_offset: f32,
    rendered_scroll_offset: f32,
    viewport_width: f32,
    viewport_height: f32,
    content_width_mode: ContentWidthMode,
    selection_color: [f32; 4],
    cursor_color: [f32; 4],
    text_color: [f32; 4],
    cursors: Vec<CursorDisplay>,
    zoom: f32,
    rendered_zoom: f32,
    /// `TextFontService::scale_generation` at the time of the last
    /// `layout_*` call. Used by
    /// [`layout_dirty_for_scale`](DocumentFlow::layout_dirty_for_scale)
    /// so the framework can detect HiDPI transitions and re-run
    /// layout without having to track them itself.
    layout_scale_generation: u64,
    /// Whether any `layout_*` call has been made at least once.
    has_layout: bool,
}

impl DocumentFlow {
    /// Create an empty flow with no content.
    ///
    /// After construction the caller typically calls
    /// [`set_viewport`](Self::set_viewport) and one of the
    /// `layout_*` methods before the first render.
    pub fn new() -> Self {
        Self {
            flow_layout: FlowLayout::new(),
            render_frame: RenderFrame::new(),
            scroll_offset: 0.0,
            rendered_scroll_offset: f32::NAN,
            viewport_width: 0.0,
            viewport_height: 0.0,
            content_width_mode: ContentWidthMode::Auto,
            selection_color: [0.26, 0.52, 0.96, 0.3],
            cursor_color: [0.0, 0.0, 0.0, 1.0],
            text_color: [0.0, 0.0, 0.0, 1.0],
            cursors: Vec::new(),
            zoom: 1.0,
            rendered_zoom: f32::NAN,
            layout_scale_generation: 0,
            has_layout: false,
        }
    }

    // ── Viewport & content width ───────────────────────────────

    /// Set the visible area dimensions in logical pixels.
    ///
    /// The viewport controls:
    ///
    /// - **Culling**: only blocks within the viewport are rendered.
    /// - **Selection highlight**: multi-line selection extends to
    ///   the viewport width.
    /// - **Layout width** (in [`ContentWidthMode::Auto`]): text
    ///   wraps at `viewport_width / zoom`.
    ///
    /// Call this when the widget's container resizes. A resize by
    /// itself does not relayout — re-run `layout_full` /
    /// `layout_blocks` if the wrap width changed.
    pub fn set_viewport(&mut self, width: f32, height: f32) {
        self.viewport_width = width;
        self.viewport_height = height;
        self.flow_layout.viewport_width = width;
        self.flow_layout.viewport_height = height;
    }

    /// Current viewport width in logical pixels.
    pub fn viewport_width(&self) -> f32 {
        self.viewport_width
    }

    /// Current viewport height in logical pixels.
    pub fn viewport_height(&self) -> f32 {
        self.viewport_height
    }

    /// Pin content width at a fixed value, independent of viewport.
    ///
    /// Text wraps at this width regardless of how wide the viewport
    /// is. Use for page-like (WYSIWYG) layout or documents with an
    /// explicit column width. Pass `f32::INFINITY` for no-wrap mode.
    pub fn set_content_width(&mut self, width: f32) {
        self.content_width_mode = ContentWidthMode::Fixed(width);
    }

    /// Reflow content width to follow the viewport (the default).
    ///
    /// Text re-wraps on every viewport resize. Standard editor and
    /// web-style layout.
    pub fn set_content_width_auto(&mut self) {
        self.content_width_mode = ContentWidthMode::Auto;
    }

    /// The effective width used for text layout (line wrapping,
    /// table columns, etc.).
    ///
    /// In [`ContentWidthMode::Auto`], equals `viewport_width / zoom`
    /// so that text reflows to fit the zoomed viewport. In
    /// [`ContentWidthMode::Fixed`], equals the set value (zoom only
    /// magnifies the rendered output).
    pub fn layout_width(&self) -> f32 {
        match self.content_width_mode {
            ContentWidthMode::Auto => self.viewport_width / self.zoom,
            ContentWidthMode::Fixed(w) => w,
        }
    }

    /// The currently configured content-width mode.
    pub fn content_width_mode(&self) -> ContentWidthMode {
        self.content_width_mode
    }

    /// Set the vertical scroll offset in logical pixels from the
    /// top of the document. Affects culling and screen-space `y`
    /// coordinates in the rendered frame.
    pub fn set_scroll_offset(&mut self, offset: f32) {
        self.scroll_offset = offset;
    }

    /// Current vertical scroll offset.
    pub fn scroll_offset(&self) -> f32 {
        self.scroll_offset
    }

    /// Total content height after layout, in logical pixels.
    pub fn content_height(&self) -> f32 {
        self.flow_layout.content_height
    }

    /// Maximum content width across all laid-out lines, in logical
    /// pixels. Used for horizontal scrollbar range when wrapping
    /// is disabled.
    pub fn max_content_width(&self) -> f32 {
        self.flow_layout.cached_max_content_width
    }

    // ── Zoom ────────────────────────────────────────────────────

    /// Set the display zoom level (PDF-style, no reflow).
    ///
    /// Zoom is a pure display transform: layout stays at base size
    /// and all screen-space output (glyph quads, decorations, caret
    /// rects) is scaled by this factor. Hit-test inputs are
    /// inversely scaled.
    ///
    /// For browser-style zoom that reflows text, combine with
    /// `set_content_width(viewport_width / zoom)`.
    ///
    /// Clamped to `0.1..=10.0`. Default is `1.0`.
    pub fn set_zoom(&mut self, zoom: f32) {
        self.zoom = zoom.clamp(0.1, 10.0);
    }

    /// Current display zoom level.
    pub fn zoom(&self) -> f32 {
        self.zoom
    }

    // ── Scale factor sync ───────────────────────────────────────

    /// Whether any `layout_*` method has run on this flow at least
    /// once. Callers that need to distinguish "never laid out"
    /// from "laid out against a stale scale factor" read this
    /// alongside [`layout_dirty_for_scale`](Self::layout_dirty_for_scale).
    pub fn has_layout(&self) -> bool {
        self.has_layout
    }

    /// Returns `true` when the backing [`TextFontService`] has had
    /// its HiDPI scale factor changed since this flow was last laid
    /// out, meaning stored shaped advances and cached ppem values
    /// are stale.
    ///
    /// Call after every `service.set_scale_factor(...)` to decide
    /// whether to re-run `layout_full` / `layout_blocks` before the
    /// next render. Returns `false` for flows that have never been
    /// laid out at all (nothing to invalidate).
    pub fn layout_dirty_for_scale(&self, service: &TextFontService) -> bool {
        self.has_layout && self.layout_scale_generation != service.scale_generation()
    }

    // ── Layout ──────────────────────────────────────────────────

    /// Full layout from a text-document `FlowSnapshot`.
    ///
    /// Clears any existing flow state and lays out every element
    /// (blocks, tables, frames) from the snapshot in flow order.
    /// Call on document load or `DocumentReset`. For single-block
    /// edits prefer [`relayout_block`](Self::relayout_block).
    #[cfg(feature = "text-document")]
    pub fn layout_full(&mut self, service: &TextFontService, flow: &text_document::FlowSnapshot) {
        use crate::bridge::convert_flow;

        let converted = convert_flow(flow);

        // Merge all elements by flow index and process in order.
        let mut all_items: Vec<(usize, FlowItemKind)> = Vec::new();
        for (idx, params) in converted.blocks {
            all_items.push((idx, FlowItemKind::Block(params)));
        }
        for (idx, params) in converted.tables {
            all_items.push((idx, FlowItemKind::Table(params)));
        }
        for (idx, params) in converted.frames {
            all_items.push((idx, FlowItemKind::Frame(params)));
        }
        all_items.sort_by_key(|(idx, _)| *idx);

        let lw = self.layout_width();
        self.flow_layout.clear();
        self.flow_layout.viewport_width = self.viewport_width;
        self.flow_layout.viewport_height = self.viewport_height;
        self.flow_layout.scale_factor = service.scale_factor;

        for (_idx, kind) in all_items {
            match kind {
                FlowItemKind::Block(params) => {
                    self.flow_layout
                        .add_block(&service.font_registry, &params, lw);
                }
                FlowItemKind::Table(params) => {
                    self.flow_layout
                        .add_table(&service.font_registry, &params, lw);
                }
                FlowItemKind::Frame(params) => {
                    self.flow_layout
                        .add_frame(&service.font_registry, &params, lw);
                }
            }
        }

        self.note_layout_done(service);
    }

    /// Lay out a list of blocks from scratch.
    ///
    /// Framework-agnostic entry point — the caller assembles
    /// [`BlockLayoutParams`] directly without going through
    /// text-document. Replaces any existing flow state.
    pub fn layout_blocks(
        &mut self,
        service: &TextFontService,
        block_params: Vec<BlockLayoutParams>,
    ) {
        self.flow_layout.scale_factor = service.scale_factor;
        self.flow_layout
            .layout_blocks(&service.font_registry, block_params, self.layout_width());
        self.note_layout_done(service);
    }

    /// Append a frame to the current flow. The frame's position
    /// (inline, float, absolute) is carried in `params`.
    pub fn add_frame(&mut self, service: &TextFontService, params: &FrameLayoutParams) {
        self.flow_layout.scale_factor = service.scale_factor;
        self.flow_layout
            .add_frame(&service.font_registry, params, self.layout_width());
        self.note_layout_done(service);
    }

    /// Append a table to the current flow.
    pub fn add_table(&mut self, service: &TextFontService, params: &TableLayoutParams) {
        self.flow_layout.scale_factor = service.scale_factor;
        self.flow_layout
            .add_table(&service.font_registry, params, self.layout_width());
        self.note_layout_done(service);
    }

    /// Relayout a single block after its content or formatting
    /// changed.
    ///
    /// Re-shapes and re-wraps just that block, then shifts
    /// subsequent items if the height changed. Much cheaper than a
    /// full layout for single-block edits (typing, format toggles).
    /// If the block lives inside a table cell, the row height is
    /// re-measured and content below the table shifts.
    ///
    /// # Invariants
    ///
    /// This is an incremental operation and only makes sense when
    /// a valid layout is already installed on this flow, laid out
    /// against the same HiDPI scale factor the service currently
    /// reports. Violations produce a [`RelayoutError`]:
    ///
    /// - [`RelayoutError::NoLayout`] if no `layout_*` method has
    ///   run on this flow yet — there is nothing to update.
    /// - [`RelayoutError::ScaleDirty`] if the service's scale
    ///   factor has changed since the last layout — reshaping a
    ///   single block would leave neighbors at the old ppem and
    ///   produce an inconsistent flow. The caller must re-run
    ///   [`layout_full`](Self::layout_full) / [`layout_blocks`](Self::layout_blocks)
    ///   first.
    ///
    /// Both conditions are detected structurally from
    /// [`has_layout`](Self::has_layout) and
    /// [`layout_dirty_for_scale`](Self::layout_dirty_for_scale),
    /// so callers that already guard those don't need to handle
    /// the error.
    pub fn relayout_block(
        &mut self,
        service: &TextFontService,
        params: &BlockLayoutParams,
    ) -> Result<(), RelayoutError> {
        if !self.has_layout {
            return Err(RelayoutError::NoLayout);
        }
        if self.layout_scale_generation != service.scale_generation() {
            return Err(RelayoutError::ScaleDirty);
        }
        self.flow_layout.scale_factor = service.scale_factor;
        self.flow_layout
            .relayout_block(&service.font_registry, params, self.layout_width());
        self.note_layout_done(service);
        Ok(())
    }

    fn note_layout_done(&mut self, service: &TextFontService) {
        self.has_layout = true;
        self.layout_scale_generation = service.scale_generation();
    }

    // ── Rendering ──────────────────────────────────────────────

    /// Render the visible viewport and return the produced frame.
    ///
    /// Performs viewport culling, rasterizes any glyphs missing
    /// from the atlas into it, and emits glyph quads, image quads,
    /// and decoration rectangles. The returned reference borrows
    /// both `self` and `service`; drop it before the next mutation.
    ///
    /// On every call, stale glyphs (unused for ~120 frames) are
    /// evicted from the atlas to reclaim slot space.
    pub fn render(&mut self, service: &mut TextFontService) -> &RenderFrame {
        let effective_vw = self.viewport_width / self.zoom;
        let effective_vh = self.viewport_height / self.zoom;
        crate::render::frame::build_render_frame(
            &self.flow_layout,
            &service.font_registry,
            &mut service.atlas,
            &mut service.glyph_cache,
            &mut service.scale_context,
            self.scroll_offset,
            effective_vw,
            effective_vh,
            &self.cursors,
            self.cursor_color,
            self.selection_color,
            self.text_color,
            &mut self.render_frame,
        );
        self.rendered_scroll_offset = self.scroll_offset;
        self.rendered_zoom = self.zoom;
        apply_zoom(&mut self.render_frame, self.zoom);
        &self.render_frame
    }

    /// Incremental render that only re-renders one block's glyphs.
    ///
    /// Reuses cached glyph / decoration data for all other blocks
    /// from the last full `render()`. Call after
    /// [`relayout_block`](Self::relayout_block) when only one block's
    /// text changed.
    ///
    /// Falls back to a full [`render`](Self::render) if the block's
    /// height changed (subsequent glyph positions would be stale),
    /// if scroll offset or zoom changed since the last full render,
    /// or if the block lives inside a table / frame (those are
    /// cached with a different key).
    pub fn render_block_only(
        &mut self,
        service: &mut TextFontService,
        block_id: usize,
    ) -> &RenderFrame {
        if (self.scroll_offset - self.rendered_scroll_offset).abs() > 0.001
            || (self.zoom - self.rendered_zoom).abs() > 0.001
        {
            return self.render(service);
        }

        if !self.flow_layout.blocks.contains_key(&block_id) {
            let in_table = self.flow_layout.tables.values().any(|table| {
                table
                    .cell_layouts
                    .iter()
                    .any(|c| c.blocks.iter().any(|b| b.block_id == block_id))
            });
            if in_table {
                return self.render(service);
            }
            let in_frame = self
                .flow_layout
                .frames
                .values()
                .any(|frame| crate::layout::flow::frame_contains_block(frame, block_id));
            if in_frame {
                return self.render(service);
            }
        }

        if let Some(block) = self.flow_layout.blocks.get(&block_id) {
            let old_height = self
                .render_frame
                .block_heights
                .get(&block_id)
                .copied()
                .unwrap_or(block.height);
            if (block.height - old_height).abs() > 0.001 {
                return self.render(service);
            }
        }

        let effective_vw = self.viewport_width / self.zoom;
        let effective_vh = self.viewport_height / self.zoom;
        let scale_factor = service.scale_factor;
        let mut new_glyphs = Vec::new();
        let mut new_images = Vec::new();
        if let Some(block) = self.flow_layout.blocks.get(&block_id) {
            let mut tmp = RenderFrame::new();
            crate::render::frame::render_block_at_offset(
                block,
                0.0,
                0.0,
                &service.font_registry,
                &mut service.atlas,
                &mut service.glyph_cache,
                &mut service.scale_context,
                self.scroll_offset,
                effective_vh,
                self.text_color,
                scale_factor,
                &mut tmp,
            );
            new_glyphs = tmp.glyphs;
            new_images = tmp.images;
        }

        let new_decos = if let Some(block) = self.flow_layout.blocks.get(&block_id) {
            crate::render::decoration::generate_block_decorations(
                block,
                &service.font_registry,
                self.scroll_offset,
                effective_vh,
                0.0,
                0.0,
                effective_vw,
                self.text_color,
                scale_factor,
            )
        } else {
            Vec::new()
        };

        if let Some(entry) = self
            .render_frame
            .block_glyphs
            .iter_mut()
            .find(|(id, _)| *id == block_id)
        {
            entry.1 = new_glyphs;
        }
        if let Some(entry) = self
            .render_frame
            .block_images
            .iter_mut()
            .find(|(id, _)| *id == block_id)
        {
            entry.1 = new_images;
        }
        if let Some(entry) = self
            .render_frame
            .block_decorations
            .iter_mut()
            .find(|(id, _)| *id == block_id)
        {
            entry.1 = new_decos;
        }

        self.rebuild_flat_frame(service);
        apply_zoom(&mut self.render_frame, self.zoom);
        &self.render_frame
    }

    /// Lightweight render that only updates cursor/selection
    /// decorations.
    ///
    /// Reuses the existing glyph quads and images from the last
    /// full `render()`. Use when only the cursor blinked or the
    /// selection changed. Falls back to a full [`render`](Self::render)
    /// if the scroll offset or zoom changed in the meantime.
    pub fn render_cursor_only(&mut self, service: &mut TextFontService) -> &RenderFrame {
        if (self.scroll_offset - self.rendered_scroll_offset).abs() > 0.001
            || (self.zoom - self.rendered_zoom).abs() > 0.001
        {
            return self.render(service);
        }

        self.render_frame.decorations.retain(|d| {
            !matches!(
                d.kind,
                DecorationKind::Cursor | DecorationKind::Selection | DecorationKind::CellSelection
            )
        });

        let effective_vw = self.viewport_width / self.zoom;
        let effective_vh = self.viewport_height / self.zoom;
        let mut cursor_decos = crate::render::cursor::generate_cursor_decorations(
            &self.flow_layout,
            &self.cursors,
            self.scroll_offset,
            self.cursor_color,
            self.selection_color,
            effective_vw,
            effective_vh,
        );
        apply_zoom_decorations(&mut cursor_decos, self.zoom);
        self.render_frame.decorations.extend(cursor_decos);

        &self.render_frame
    }

    fn rebuild_flat_frame(&mut self, service: &mut TextFontService) {
        self.render_frame.glyphs.clear();
        self.render_frame.images.clear();
        self.render_frame.decorations.clear();
        for (_, glyphs) in &self.render_frame.block_glyphs {
            self.render_frame.glyphs.extend_from_slice(glyphs);
        }
        for (_, images) in &self.render_frame.block_images {
            self.render_frame.images.extend_from_slice(images);
        }
        for (_, decos) in &self.render_frame.block_decorations {
            self.render_frame.decorations.extend_from_slice(decos);
        }

        for item in &self.flow_layout.flow_order {
            match item {
                FlowItem::Table { table_id, .. } => {
                    if let Some(table) = self.flow_layout.tables.get(table_id) {
                        let decos = crate::layout::table::generate_table_decorations(
                            table,
                            self.scroll_offset,
                        );
                        self.render_frame.decorations.extend(decos);
                    }
                }
                FlowItem::Frame { frame_id, .. } => {
                    if let Some(frame) = self.flow_layout.frames.get(frame_id) {
                        crate::render::frame::append_frame_border_decorations(
                            frame,
                            self.scroll_offset,
                            &mut self.render_frame.decorations,
                        );
                    }
                }
                FlowItem::Block { .. } => {}
            }
        }

        let effective_vw = self.viewport_width / self.zoom;
        let effective_vh = self.viewport_height / self.zoom;
        let cursor_decos = crate::render::cursor::generate_cursor_decorations(
            &self.flow_layout,
            &self.cursors,
            self.scroll_offset,
            self.cursor_color,
            self.selection_color,
            effective_vw,
            effective_vh,
        );
        self.render_frame.decorations.extend(cursor_decos);

        self.render_frame.atlas_dirty = service.atlas.dirty;
        self.render_frame.atlas_width = service.atlas.width;
        self.render_frame.atlas_height = service.atlas.height;
        if service.atlas.dirty {
            let pixels = &service.atlas.pixels;
            let needed = (service.atlas.width * service.atlas.height * 4) as usize;
            self.render_frame.atlas_pixels.resize(needed, 0);
            let copy_len = needed.min(pixels.len());
            self.render_frame.atlas_pixels[..copy_len].copy_from_slice(&pixels[..copy_len]);
            service.atlas.dirty = false;
        }
    }

    // ── Single-line layout ──────────────────────────────────────

    /// Lay out a single line of text and return GPU-ready glyph
    /// quads. Fast path for labels, tooltips, overlays — anything
    /// that doesn't need the full document pipeline.
    ///
    /// If `max_width` is set and the shaped text exceeds it, the
    /// output is truncated with an ellipsis character. Glyph quads
    /// are positioned with the top-left at `(0, 0)`.
    pub fn layout_single_line(
        &mut self,
        service: &mut TextFontService,
        text: &str,
        format: &TextFormat,
        max_width: Option<f32>,
    ) -> SingleLineResult {
        let empty = SingleLineResult {
            width: 0.0,
            height: 0.0,
            baseline: 0.0,
            glyphs: Vec::new(),
            spans: Vec::new(),
        };

        if text.is_empty() {
            return empty;
        }

        let font_point_size = format.font_size.map(|s| s as u32);
        let resolved = match resolve_font(
            &service.font_registry,
            format.font_family.as_deref(),
            format.font_weight,
            format.font_bold,
            format.font_italic,
            font_point_size,
            service.scale_factor,
        ) {
            Some(r) => r,
            None => return empty,
        };

        let metrics = match font_metrics_px(&service.font_registry, &resolved) {
            Some(m) => m,
            None => return empty,
        };
        let line_height = metrics.ascent + metrics.descent + metrics.leading;
        let baseline = metrics.ascent;

        let runs: Vec<_> = bidi_runs(text)
            .into_iter()
            .filter_map(|br| {
                let slice = text.get(br.byte_range.clone())?;
                shape_text_with_fallback(
                    &service.font_registry,
                    &resolved,
                    slice,
                    br.byte_range.start,
                    br.direction,
                )
            })
            .collect();

        if runs.is_empty() {
            return empty;
        }

        let total_advance: f32 = runs.iter().map(|r| r.advance_width).sum();

        let (truncate_at_visual_index, final_width, ellipsis_run) = if let Some(max_w) = max_width
            && total_advance > max_w
        {
            let ellipsis_run = shape_text(&service.font_registry, &resolved, "\u{2026}", 0);
            let ellipsis_width = ellipsis_run
                .as_ref()
                .map(|r| r.advance_width)
                .unwrap_or(0.0);
            let budget = (max_w - ellipsis_width).max(0.0);

            let mut used = 0.0f32;
            let mut count = 0usize;
            'outer: for run in &runs {
                for g in &run.glyphs {
                    if used + g.x_advance > budget {
                        break 'outer;
                    }
                    used += g.x_advance;
                    count += 1;
                }
            }

            (Some(count), used + ellipsis_width, ellipsis_run)
        } else {
            (None, total_advance, None)
        };

        let text_color = format.color.unwrap_or(self.text_color);
        let glyph_capacity: usize = runs.iter().map(|r| r.glyphs.len()).sum();
        let mut quads = Vec::with_capacity(glyph_capacity + 1);
        let mut pen_x = 0.0f32;
        let mut emitted = 0usize;

        'emit: for run in &runs {
            for glyph in &run.glyphs {
                if let Some(limit) = truncate_at_visual_index
                    && emitted >= limit
                {
                    break 'emit;
                }
                rasterize_glyph_quad(service, glyph, run, pen_x, baseline, text_color, &mut quads);
                pen_x += glyph.x_advance;
                emitted += 1;
            }
        }

        if let Some(ref e_run) = ellipsis_run {
            for glyph in &e_run.glyphs {
                rasterize_glyph_quad(
                    service, glyph, e_run, pen_x, baseline, text_color, &mut quads,
                );
                pen_x += glyph.x_advance;
            }
        }

        SingleLineResult {
            width: final_width,
            height: line_height,
            baseline,
            glyphs: quads,
            spans: Vec::new(),
        }
    }

    /// Lay out a multi-line paragraph by wrapping text at `max_width`.
    ///
    /// Multi-line counterpart to
    /// [`layout_single_line`](Self::layout_single_line). Shapes the
    /// input, breaks it at Unicode line-break opportunities
    /// (greedy, left-aligned), and rasterizes each line's glyphs
    /// into paragraph-local coordinates starting at `(0, 0)`.
    ///
    /// If `max_lines` is `Some(n)`, at most `n` lines are emitted
    /// and any remainder is silently dropped.
    pub fn layout_paragraph(
        &mut self,
        service: &mut TextFontService,
        text: &str,
        format: &TextFormat,
        max_width: f32,
        max_lines: Option<usize>,
    ) -> ParagraphResult {
        let empty = ParagraphResult {
            width: 0.0,
            height: 0.0,
            baseline_first: 0.0,
            line_count: 0,
            line_height: 0.0,
            glyphs: Vec::new(),
            spans: Vec::new(),
        };

        if text.is_empty() || max_width <= 0.0 {
            return empty;
        }

        let font_point_size = format.font_size.map(|s| s as u32);
        let resolved = match resolve_font(
            &service.font_registry,
            format.font_family.as_deref(),
            format.font_weight,
            format.font_bold,
            format.font_italic,
            font_point_size,
            service.scale_factor,
        ) {
            Some(r) => r,
            None => return empty,
        };

        let metrics = match font_metrics_px(&service.font_registry, &resolved) {
            Some(m) => m,
            None => return empty,
        };

        let runs: Vec<_> = bidi_runs(text)
            .into_iter()
            .filter_map(|br| {
                let slice = text.get(br.byte_range.clone())?;
                shape_text_with_fallback(
                    &service.font_registry,
                    &resolved,
                    slice,
                    br.byte_range.start,
                    br.direction,
                )
            })
            .collect();

        if runs.is_empty() {
            return empty;
        }

        let lines = break_into_lines(runs, text, max_width, Alignment::Left, 0.0, &metrics);

        let line_count = match max_lines {
            Some(n) => lines.len().min(n),
            None => lines.len(),
        };

        let text_color = format.color.unwrap_or(self.text_color);
        let mut quads: Vec<GlyphQuad> = Vec::new();
        let mut y_top = 0.0f32;
        let mut max_line_width = 0.0f32;
        let baseline_first = metrics.ascent;

        for line in lines.iter().take(line_count) {
            if line.width > max_line_width {
                max_line_width = line.width;
            }
            let baseline_y = y_top + metrics.ascent;
            for run in &line.runs {
                let mut pen_x = run.x;
                let run_copy = run.shaped_run.clone();
                for glyph in &run_copy.glyphs {
                    rasterize_glyph_quad(
                        service, glyph, &run_copy, pen_x, baseline_y, text_color, &mut quads,
                    );
                    pen_x += glyph.x_advance;
                }
            }
            y_top += metrics.ascent + metrics.descent + metrics.leading;
        }

        let line_height = metrics.ascent + metrics.descent + metrics.leading;
        ParagraphResult {
            width: max_line_width,
            height: y_top,
            baseline_first,
            line_count,
            line_height,
            glyphs: quads,
            spans: Vec::new(),
        }
    }

    /// Single-line layout with inline markup. See
    /// [`layout_single_line`](Self::layout_single_line) for the plain
    /// variant. Accepts parsed `[label](url)`, `*italic*`, and
    /// `**bold**` spans and annotates the output with per-span
    /// bounding rectangles for hit-testing.
    pub fn layout_single_line_markup(
        &mut self,
        service: &mut TextFontService,
        markup: &InlineMarkup,
        format: &TextFormat,
        max_width: Option<f32>,
    ) -> SingleLineResult {
        if markup.spans.is_empty() {
            return SingleLineResult {
                width: 0.0,
                height: 0.0,
                baseline: 0.0,
                glyphs: Vec::new(),
                spans: Vec::new(),
            };
        }

        let per_span: Vec<(SingleLineResult, &crate::layout::inline_markup::InlineSpan)> = markup
            .spans
            .iter()
            .map(|sp| {
                let fmt = merge_format(format, sp.attrs);
                let r = if sp.text.is_empty() {
                    SingleLineResult {
                        width: 0.0,
                        height: 0.0,
                        baseline: 0.0,
                        glyphs: Vec::new(),
                        spans: Vec::new(),
                    }
                } else {
                    self.layout_single_line(service, &sp.text, &fmt, None)
                };
                (r, sp)
            })
            .collect();

        let total_width: f32 = per_span.iter().map(|(r, _)| r.width).sum();
        let line_height = per_span
            .iter()
            .map(|(r, _)| r.height)
            .fold(0.0f32, f32::max);
        let baseline = per_span
            .iter()
            .map(|(r, _)| r.baseline)
            .fold(0.0f32, f32::max);

        let truncate = match max_width {
            Some(mw) if total_width > mw => Some(mw),
            _ => None,
        };

        let mut glyphs: Vec<GlyphQuad> = Vec::new();
        let mut spans_out: Vec<LaidOutSpan> = Vec::new();
        let mut pen_x: f32 = 0.0;
        let effective_width = truncate.unwrap_or(total_width);

        for (r, sp) in &per_span {
            let remaining = (effective_width - pen_x).max(0.0);
            let span_visible_width = r.width.min(remaining);
            if span_visible_width <= 0.0 && r.width > 0.0 {
                spans_out.push(LaidOutSpan {
                    kind: if let Some(url) = sp.link_url.clone() {
                        LaidOutSpanKind::Link { url }
                    } else {
                        LaidOutSpanKind::Text
                    },
                    line_index: 0,
                    rect: [pen_x, 0.0, 0.0, line_height],
                    byte_range: sp.byte_range.clone(),
                });
                continue;
            }

            for g in &r.glyphs {
                let g_right = pen_x + g.screen[0] + g.screen[2];
                if g_right > effective_width + 0.5 {
                    break;
                }
                let mut gq = g.clone();
                gq.screen[0] += pen_x;
                glyphs.push(gq);
            }

            spans_out.push(LaidOutSpan {
                kind: if let Some(url) = sp.link_url.clone() {
                    LaidOutSpanKind::Link { url }
                } else {
                    LaidOutSpanKind::Text
                },
                line_index: 0,
                rect: [pen_x, 0.0, span_visible_width, line_height],
                byte_range: sp.byte_range.clone(),
            });

            pen_x += r.width;
            if truncate.is_some() && pen_x >= effective_width {
                break;
            }
        }

        SingleLineResult {
            width: effective_width,
            height: line_height,
            baseline,
            glyphs,
            spans: spans_out,
        }
    }

    /// Paragraph layout with inline markup. Multi-line counterpart
    /// to [`layout_single_line_markup`](Self::layout_single_line_markup).
    /// Emits a [`LaidOutSpan`] for every link segment so the caller
    /// can hit-test against wrapped links.
    pub fn layout_paragraph_markup(
        &mut self,
        service: &mut TextFontService,
        markup: &InlineMarkup,
        format: &TextFormat,
        max_width: f32,
        max_lines: Option<usize>,
    ) -> ParagraphResult {
        let empty = ParagraphResult {
            width: 0.0,
            height: 0.0,
            baseline_first: 0.0,
            line_count: 0,
            line_height: 0.0,
            glyphs: Vec::new(),
            spans: Vec::new(),
        };

        if markup.spans.is_empty() || max_width <= 0.0 {
            return empty;
        }

        let mut flat = String::new();
        let mut span_flat_offsets: Vec<usize> = Vec::with_capacity(markup.spans.len());
        for sp in &markup.spans {
            span_flat_offsets.push(flat.len());
            flat.push_str(&sp.text);
        }
        if flat.is_empty() {
            return empty;
        }

        let base_point_size = format.font_size.map(|s| s as u32);
        let base_resolved = match resolve_font(
            &service.font_registry,
            format.font_family.as_deref(),
            format.font_weight,
            format.font_bold,
            format.font_italic,
            base_point_size,
            service.scale_factor,
        ) {
            Some(r) => r,
            None => return empty,
        };
        let metrics = match font_metrics_px(&service.font_registry, &base_resolved) {
            Some(m) => m,
            None => return empty,
        };

        let mut all_runs: Vec<ShapedRun> = Vec::new();
        for (span_idx, sp) in markup.spans.iter().enumerate() {
            if sp.text.is_empty() {
                continue;
            }
            let fmt = merge_format(format, sp.attrs);
            let span_point_size = fmt.font_size.map(|s| s as u32);
            let Some(resolved) = resolve_font(
                &service.font_registry,
                fmt.font_family.as_deref(),
                fmt.font_weight,
                fmt.font_bold,
                fmt.font_italic,
                span_point_size,
                service.scale_factor,
            ) else {
                continue;
            };

            let flat_start = span_flat_offsets[span_idx];
            for br in bidi_runs(&sp.text) {
                let slice = match sp.text.get(br.byte_range.clone()) {
                    Some(s) => s,
                    None => continue,
                };
                let Some(mut run) = shape_text_with_fallback(
                    &service.font_registry,
                    &resolved,
                    slice,
                    flat_start + br.byte_range.start,
                    br.direction,
                ) else {
                    continue;
                };
                if let Some(url) = sp.link_url.as_ref() {
                    run.is_link = true;
                    run.anchor_href = Some(url.clone());
                }
                all_runs.push(run);
            }
        }

        if all_runs.is_empty() {
            return empty;
        }

        let lines = break_into_lines(all_runs, &flat, max_width, Alignment::Left, 0.0, &metrics);

        let line_count = match max_lines {
            Some(n) => lines.len().min(n),
            None => lines.len(),
        };

        let text_color = format.color.unwrap_or(self.text_color);
        let mut glyphs_out: Vec<GlyphQuad> = Vec::new();
        let mut spans_out: Vec<LaidOutSpan> = Vec::new();
        let line_height = metrics.ascent + metrics.descent + metrics.leading;
        let mut y_top: f32 = 0.0;
        let mut max_line_width: f32 = 0.0;
        let baseline_first = metrics.ascent;

        for (line_idx, line) in lines.iter().take(line_count).enumerate() {
            if line.width > max_line_width {
                max_line_width = line.width;
            }
            let baseline_y = y_top + metrics.ascent;

            for pr in &line.runs {
                let run_copy = pr.shaped_run.clone();
                let mut pen_x = pr.x;
                for glyph in &run_copy.glyphs {
                    rasterize_glyph_quad(
                        service,
                        glyph,
                        &run_copy,
                        pen_x,
                        baseline_y,
                        text_color,
                        &mut glyphs_out,
                    );
                    pen_x += glyph.x_advance;
                }

                if pr.decorations.is_link
                    && let Some(url) = pr.decorations.anchor_href.clone()
                {
                    let width = pr.shaped_run.advance_width;
                    spans_out.push(LaidOutSpan {
                        kind: LaidOutSpanKind::Link { url },
                        line_index: line_idx,
                        rect: [pr.x, y_top, width, line_height],
                        byte_range: pr.shaped_run.text_range.clone(),
                    });
                }
            }

            y_top += line_height;
        }

        ParagraphResult {
            width: max_line_width,
            height: y_top,
            baseline_first,
            line_count,
            line_height,
            glyphs: glyphs_out,
            spans: spans_out,
        }
    }

    // ── Hit testing & character geometry ───────────────────────

    /// Map a screen-space point to a document position. Coordinates
    /// are relative to the widget's top-left corner; the scroll
    /// offset is applied internally. Returns `None` when the flow
    /// has no content.
    pub fn hit_test(&self, x: f32, y: f32) -> Option<HitTestResult> {
        crate::render::hit_test::hit_test(
            &self.flow_layout,
            self.scroll_offset,
            x / self.zoom,
            y / self.zoom,
        )
    }

    /// Per-character advance geometry within a laid-out block.
    ///
    /// Used by accessibility layers that need to expose character
    /// positions to screen readers (AccessKit's `character_positions`
    /// / `character_widths` on `Role::TextRun`). `char_start` and
    /// `char_end` are block-relative character offsets. Returns one
    /// entry per character in the range, with `position` measured
    /// in run-local coordinates (the first character sits at `0`).
    pub fn character_geometry(
        &self,
        block_id: usize,
        char_start: usize,
        char_end: usize,
    ) -> Vec<CharacterGeometry> {
        if char_start >= char_end {
            return Vec::new();
        }
        let block = match self.flow_layout.blocks.get(&block_id) {
            Some(b) => b,
            None => return Vec::new(),
        };

        let mut absolute: Vec<(usize, f32)> = Vec::with_capacity(char_end - char_start);
        for line in &block.lines {
            if line.char_range.end <= char_start || line.char_range.start >= char_end {
                continue;
            }
            let local_start = char_start.max(line.char_range.start);
            let local_end = char_end.min(line.char_range.end);
            for c in local_start..local_end {
                let x = line.x_for_offset(c);
                absolute.push((c, x));
            }
            if local_end == char_end {
                let x_end = line.x_for_offset(local_end);
                absolute.push((local_end, x_end));
            }
        }

        if absolute.is_empty() {
            return Vec::new();
        }

        absolute.sort_by_key(|(c, _)| *c);

        let base_x = absolute.first().map(|(_, x)| *x).unwrap_or(0.0);
        let mut out: Vec<CharacterGeometry> = Vec::with_capacity(absolute.len());
        for window in absolute.windows(2) {
            let (c, x) = window[0];
            let (_, x_next) = window[1];
            if c >= char_end {
                break;
            }
            out.push(CharacterGeometry {
                position: x - base_x,
                width: (x_next - x).max(0.0),
            });
        }
        out
    }

    /// Screen-space caret rectangle at a document position, as
    /// `[x, y, width, height]`. Feed this to the platform IME for
    /// composition window placement. For drawing the caret itself,
    /// use the `DecorationKind::Cursor` entry in
    /// [`RenderFrame::decorations`] instead.
    pub fn caret_rect(&self, position: usize) -> [f32; 4] {
        let mut rect =
            crate::render::hit_test::caret_rect(&self.flow_layout, self.scroll_offset, position);
        rect[0] *= self.zoom;
        rect[1] *= self.zoom;
        rect[2] *= self.zoom;
        rect[3] *= self.zoom;
        rect
    }

    // ── Cursor & colors ────────────────────────────────────────

    /// Replace the cursor display with a single cursor.
    pub fn set_cursor(&mut self, cursor: &CursorDisplay) {
        self.cursors = vec![CursorDisplay {
            position: cursor.position,
            anchor: cursor.anchor,
            visible: cursor.visible,
            selected_cells: cursor.selected_cells.clone(),
        }];
    }

    /// Replace the cursor display with multiple cursors (multi-caret
    /// editing). Each cursor independently generates a caret and
    /// optional selection highlight.
    pub fn set_cursors(&mut self, cursors: &[CursorDisplay]) {
        self.cursors = cursors
            .iter()
            .map(|c| CursorDisplay {
                position: c.position,
                anchor: c.anchor,
                visible: c.visible,
                selected_cells: c.selected_cells.clone(),
            })
            .collect();
    }

    /// Set the selection highlight color `[r, g, b, a]` in 0..=1
    /// space. Default: `[0.26, 0.52, 0.96, 0.3]` (translucent blue).
    pub fn set_selection_color(&mut self, color: [f32; 4]) {
        self.selection_color = color;
    }

    /// Set the caret color `[r, g, b, a]`. Default: black.
    pub fn set_cursor_color(&mut self, color: [f32; 4]) {
        self.cursor_color = color;
    }

    /// Set the default text color `[r, g, b, a]`, used when a
    /// fragment has no explicit `foreground_color`. Default: black.
    pub fn set_text_color(&mut self, color: [f32; 4]) {
        self.text_color = color;
    }

    /// Current default text color.
    pub fn text_color(&self) -> [f32; 4] {
        self.text_color
    }

    // ── Scrolling helpers ──────────────────────────────────────

    /// Visual position and height of a laid-out block. Returns
    /// `None` if `block_id` is not in the current layout.
    pub fn block_visual_info(&self, block_id: usize) -> Option<BlockVisualInfo> {
        let block = self.flow_layout.blocks.get(&block_id)?;
        Some(BlockVisualInfo {
            block_id,
            y: block.y,
            height: block.height,
        })
    }

    /// Whether a block lives inside any table cell.
    pub fn is_block_in_table(&self, block_id: usize) -> bool {
        self.flow_layout.tables.values().any(|table| {
            table
                .cell_layouts
                .iter()
                .any(|cell| cell.blocks.iter().any(|b| b.block_id == block_id))
        })
    }

    /// Scroll so that `position` is visible, placing it roughly one
    /// third from the top of the viewport. Returns the new offset.
    pub fn scroll_to_position(&mut self, position: usize) -> f32 {
        let rect =
            crate::render::hit_test::caret_rect(&self.flow_layout, self.scroll_offset, position);
        let target_y = rect[1] + self.scroll_offset - self.viewport_height / (3.0 * self.zoom);
        self.scroll_offset = target_y.max(0.0);
        self.scroll_offset
    }

    /// Scroll the minimum amount needed to make the current caret
    /// visible. Call after arrow-key / click / typing. Returns
    /// `Some(new_offset)` if the scroll moved, `None` otherwise.
    pub fn ensure_caret_visible(&mut self) -> Option<f32> {
        if self.cursors.is_empty() {
            return None;
        }
        let pos = self.cursors[0].position;
        let rect = crate::render::hit_test::caret_rect(&self.flow_layout, self.scroll_offset, pos);
        let caret_screen_y = rect[1];
        let caret_screen_bottom = caret_screen_y + rect[3];
        let effective_vh = self.viewport_height / self.zoom;
        let margin = 10.0 / self.zoom;
        let old_offset = self.scroll_offset;

        if caret_screen_y < 0.0 {
            self.scroll_offset += caret_screen_y - margin;
            self.scroll_offset = self.scroll_offset.max(0.0);
        } else if caret_screen_bottom > effective_vh {
            self.scroll_offset += caret_screen_bottom - effective_vh + margin;
        }

        if (self.scroll_offset - old_offset).abs() > 0.001 {
            Some(self.scroll_offset)
        } else {
            None
        }
    }
}

impl Default for DocumentFlow {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(feature = "text-document")]
enum FlowItemKind {
    Block(BlockLayoutParams),
    Table(TableLayoutParams),
    Frame(FrameLayoutParams),
}

/// Rasterize a single glyph into the service's atlas and append a
/// `GlyphQuad` to the output vec. Shared between
/// [`DocumentFlow::layout_single_line`] and
/// [`DocumentFlow::layout_paragraph`] (plus the markup variants).
fn rasterize_glyph_quad(
    service: &mut TextFontService,
    glyph: &ShapedGlyph,
    run: &ShapedRun,
    pen_x: f32,
    baseline: f32,
    text_color: [f32; 4],
    quads: &mut Vec<GlyphQuad>,
) {
    use crate::atlas::cache::GlyphCacheKey;
    use crate::atlas::rasterizer::rasterize_glyph;

    if glyph.glyph_id == 0 {
        return;
    }

    let entry = match service.font_registry.get(glyph.font_face_id) {
        Some(e) => e,
        None => return,
    };

    let sf = service.scale_factor.max(f32::MIN_POSITIVE);
    let inv_sf = 1.0 / sf;
    let physical_size_px = run.size_px * sf;
    let cache_key = GlyphCacheKey::new(glyph.font_face_id, glyph.glyph_id, physical_size_px);

    if service.glyph_cache.peek(&cache_key).is_none()
        && let Some(image) = rasterize_glyph(
            &mut service.scale_context,
            &entry.data,
            entry.face_index,
            entry.swash_cache_key,
            glyph.glyph_id,
            physical_size_px,
        )
        && image.width > 0
        && image.height > 0
        && let Some(alloc) = service.atlas.allocate(image.width, image.height)
    {
        let rect = alloc.rectangle;
        let atlas_x = rect.min.x as u32;
        let atlas_y = rect.min.y as u32;
        if image.is_color {
            service
                .atlas
                .blit_rgba(atlas_x, atlas_y, image.width, image.height, &image.data);
        } else {
            service
                .atlas
                .blit_mask(atlas_x, atlas_y, image.width, image.height, &image.data);
        }
        service.glyph_cache.insert(
            cache_key,
            crate::atlas::cache::CachedGlyph {
                alloc_id: alloc.id,
                atlas_x,
                atlas_y,
                width: image.width,
                height: image.height,
                placement_left: image.placement_left,
                placement_top: image.placement_top,
                is_color: image.is_color,
                last_used: 0,
            },
        );
    }

    if let Some(cached) = service.glyph_cache.get(&cache_key) {
        let logical_w = cached.width as f32 * inv_sf;
        let logical_h = cached.height as f32 * inv_sf;
        let logical_left = cached.placement_left as f32 * inv_sf;
        let logical_top = cached.placement_top as f32 * inv_sf;
        let screen_x = pen_x + glyph.x_offset + logical_left;
        let screen_y = baseline - glyph.y_offset - logical_top;
        let color = if cached.is_color {
            [1.0, 1.0, 1.0, 1.0]
        } else {
            text_color
        };
        quads.push(GlyphQuad {
            screen: [screen_x, screen_y, logical_w, logical_h],
            atlas: [
                cached.atlas_x as f32,
                cached.atlas_y as f32,
                cached.width as f32,
                cached.height as f32,
            ],
            color,
        });
    }
}

/// Scale all screen-space coordinates in a RenderFrame by `zoom`.
fn apply_zoom(frame: &mut RenderFrame, zoom: f32) {
    if (zoom - 1.0).abs() <= f32::EPSILON {
        return;
    }
    for q in &mut frame.glyphs {
        q.screen[0] *= zoom;
        q.screen[1] *= zoom;
        q.screen[2] *= zoom;
        q.screen[3] *= zoom;
    }
    for q in &mut frame.images {
        q.screen[0] *= zoom;
        q.screen[1] *= zoom;
        q.screen[2] *= zoom;
        q.screen[3] *= zoom;
    }
    apply_zoom_decorations(&mut frame.decorations, zoom);
}

/// Scale all screen-space coordinates in decoration rects by `zoom`.
fn apply_zoom_decorations(decorations: &mut [DecorationRect], zoom: f32) {
    if (zoom - 1.0).abs() <= f32::EPSILON {
        return;
    }
    for d in decorations.iter_mut() {
        d.rect[0] *= zoom;
        d.rect[1] *= zoom;
        d.rect[2] *= zoom;
        d.rect[3] *= zoom;
    }
}

/// Derive a per-span [`TextFormat`] from a base format and inline
/// markup attributes (bold / italic).
fn merge_format(base: &TextFormat, attrs: InlineAttrs) -> TextFormat {
    let mut fmt = base.clone();
    if attrs.is_bold() {
        fmt.font_bold = Some(true);
        if let Some(w) = fmt.font_weight
            && w < 600
        {
            fmt.font_weight = Some(700);
        } else if fmt.font_weight.is_none() {
            fmt.font_weight = Some(700);
        }
    }
    if attrs.is_italic() {
        fmt.font_italic = Some(true);
    }
    fmt
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::layout::block::{BlockLayoutParams, FragmentParams};
    use crate::layout::paragraph::Alignment;
    use crate::types::{UnderlineStyle, VerticalAlignment};

    const NOTO_SANS: &[u8] = include_bytes!("../test-fonts/NotoSans-Variable.ttf");

    fn service() -> TextFontService {
        let mut s = TextFontService::new();
        let face = s.register_font(NOTO_SANS);
        s.set_default_font(face, 16.0);
        s
    }

    fn block(id: usize, text: &str) -> BlockLayoutParams {
        BlockLayoutParams {
            block_id: id,
            position: 0,
            text: text.to_string(),
            fragments: vec![FragmentParams {
                text: text.to_string(),
                offset: 0,
                length: text.len(),
                font_family: None,
                font_weight: None,
                font_bold: None,
                font_italic: None,
                font_point_size: None,
                underline_style: UnderlineStyle::None,
                overline: false,
                strikeout: false,
                is_link: false,
                letter_spacing: 0.0,
                word_spacing: 0.0,
                foreground_color: None,
                underline_color: None,
                background_color: None,
                anchor_href: None,
                tooltip: None,
                vertical_alignment: VerticalAlignment::Normal,
                image_name: None,
                image_width: 0.0,
                image_height: 0.0,
            }],
            alignment: Alignment::Left,
            top_margin: 0.0,
            bottom_margin: 0.0,
            left_margin: 0.0,
            right_margin: 0.0,
            text_indent: 0.0,
            list_marker: String::new(),
            list_indent: 0.0,
            tab_positions: vec![],
            line_height_multiplier: None,
            non_breakable_lines: false,
            checkbox: None,
            background_color: None,
        }
    }

    #[test]
    fn relayout_block_returns_no_layout_when_never_laid_out() {
        let svc = service();
        let mut flow = DocumentFlow::new();
        flow.set_viewport(400.0, 200.0);
        let err = flow.relayout_block(&svc, &block(1, "Hello")).unwrap_err();
        assert_eq!(err, RelayoutError::NoLayout);
    }

    #[test]
    fn relayout_block_returns_scale_dirty_after_scale_factor_change() {
        let mut svc = service();
        let mut flow = DocumentFlow::new();
        flow.set_viewport(400.0, 200.0);
        flow.layout_blocks(&svc, vec![block(1, "Hello")]);
        assert!(flow.has_layout());

        // Simulate a HiDPI transition on the shared service.
        svc.set_scale_factor(2.0);
        assert!(flow.layout_dirty_for_scale(&svc));

        let err = flow
            .relayout_block(&svc, &block(1, "Hello world"))
            .unwrap_err();
        assert_eq!(err, RelayoutError::ScaleDirty);
    }

    #[test]
    fn relayout_block_succeeds_after_fresh_layout_post_scale_change() {
        let mut svc = service();
        let mut flow = DocumentFlow::new();
        flow.set_viewport(400.0, 200.0);
        flow.layout_blocks(&svc, vec![block(1, "Hello")]);

        svc.set_scale_factor(2.0);
        // Caller is expected to re-run a full layout at the new
        // scale before issuing incremental updates.
        flow.layout_blocks(&svc, vec![block(1, "Hello")]);
        assert!(!flow.layout_dirty_for_scale(&svc));

        // Now the incremental path succeeds.
        flow.relayout_block(&svc, &block(1, "Hello world"))
            .expect("relayout_block must succeed after a fresh post-scale layout");
    }
}