Struct ChatViewport 

pub struct ChatViewport {
    pub scroll_offset: usize,
    pub scroll_target: usize,
    pub scroll_pos: f32,
    pub scrollbar_thumb_top: f32,
    pub scrollbar_thumb_size: f32,
    pub auto_scroll: bool,
    pub width: u16,
    pub message_heights: Vec<usize>,
    pub message_heights_width: u16,
    pub dirty_from: Option<usize>,
    pub height_prefix_sums: Vec<usize>,
    pub prefix_sums_width: u16,
}

Single owner of all chat layout state: scroll, per-message heights, and prefix sums.

Consolidates state previously scattered across App (scroll fields, prefix sums), ChatMessage (cached_visual_height/cached_visual_width), and BlockCache (wrapped_height/wrapped_width). Per-block heights remain on BlockCache via set_height() / height_at(), but the viewport owns the validity width that governs whether those caches are considered current. On resize, on_frame() zeroes message heights and clears prefix sums, causing the next update_visual_heights() pass to re-measure every message using ground-truth Paragraph::line_count().

Fields

scroll_offset: usize

Rendered scroll offset (rounded from scroll_pos).

scroll_target: usize

Target scroll offset requested by user input or auto-scroll.

scroll_pos: f32

Smooth scroll position (fractional) for animation.

scrollbar_thumb_top: f32

Smoothed scrollbar thumb top row (fractional) for animation.

scrollbar_thumb_size: f32

Smoothed scrollbar thumb height (fractional) for animation.

auto_scroll: bool

Whether to auto-scroll to bottom on new content.

width: u16

Current terminal width. Set by on_frame() each render cycle.

message_heights: Vec<usize>

Visual height (in terminal rows) of each message, indexed by message position. Zeroed on resize; rebuilt by measure_message_height().

message_heights_width: u16

Width at which message_heights was last computed.

dirty_from: Option<usize>

Oldest message index whose cached height may be stale.

height_prefix_sums: Vec<usize>

Cumulative heights: height_prefix_sums[i] = sum of heights 0..=i. Enables O(log n) binary search for first visible message and O(1) total height.

prefix_sums_width: u16

Width at which prefix sums were last computed.

Implementations

impl ChatViewport

pub fn new() -> Self

Create a new viewport with default scroll state (auto-scroll enabled).

pub fn on_frame(&mut self, width: u16)

Called at top of each render frame. Detects width change and invalidates all cached heights so they get re-measured at the new width.

pub fn message_height(&self, idx: usize) -> usize

Get the cached visual height for message idx. Returns 0 if not yet computed.

pub fn set_message_height(&mut self, idx: usize, h: usize)

Set the visual height for message idx, growing the vec if needed.

Does NOT update message_heights_width - the caller must call mark_heights_valid() after the full re-measurement pass completes.

pub fn mark_heights_valid(&mut self)

Mark all message heights as valid at the current width. Call after update_visual_heights() finishes re-measuring.

pub fn mark_message_dirty(&mut self, idx: usize)

Mark cached heights dirty from idx onward.

pub fn rebuild_prefix_sums(&mut self)

Rebuild prefix sums from message_heights. O(1) fast path when width unchanged and only the last message changed (streaming).

pub fn total_message_height(&self) -> usize

Total height of all messages (O(1) via prefix sums).

pub fn cumulative_height_before(&self, idx: usize) -> usize

Cumulative height of messages 0..idx (O(1) via prefix sums).

pub fn find_first_visible(&self, scroll_offset: usize) -> usize

Binary search for the first message whose cumulative range overlaps scroll_offset.

pub fn scroll_up(&mut self, lines: usize)

Scroll up by lines. Disables auto-scroll.

pub fn scroll_down(&mut self, lines: usize)

Scroll down by lines. Auto-scroll re-engagement handled by render.

pub fn engage_auto_scroll(&mut self)

Re-engage auto-scroll (stick to bottom).

Trait Implementations

impl Default for ChatViewport

fn default() -> Self

Returns the “default value” for a type.