Task List Checkbox Rendering

Interactive task list checkboxes in the rendered markdown preview.

Overview

Task lists (GitHub-style - [ ] and - [x] syntax) are rendered as interactive egui Checkbox widgets in the rendered and split views. Clicking a checkbox toggles the task state and updates the underlying markdown source.

Key Files

File	Purpose
src/markdown/editor.rs	Checkbox rendering in render_list_item() and render_list_item_with_structural_keys()
src/markdown/parser.rs	Task item detection in AST (MarkdownNodeType::TaskItem)

Implementation Details

Rendering

Task list items are detected during AST traversal. When a task item is found:

1. Checkbox UI: Renders egui::Checkbox instead of ASCII [ ]/[x] text
2. Click Handling: Captures checkbox response with ui.checkbox(&mut checked, "")
3. Source Toggle: On click, toggles the source line between [ ] and [x]
4. State Tracking: Changes are marked in edit_state for markdown rebuild

Code Pattern

if is_task {
    let mut checked = task_checked;
    let checkbox_response = ui.checkbox(&mut checked, "");

    if checkbox_response.changed() {
        // Toggle source line
        if let Some(source_line) = source.lines().nth(node.start_line.saturating_sub(1)) {
            let new_line = if task_checked {
                source_line.replace("[x]", "[ ]").replace("[X]", "[ ]")
            } else {
                source_line.replace("[ ]", "[x]")
            };
            update_source_line(source, node.start_line, &new_line);
            
            // Mark modified
            let node_id = edit_state.add_node(...);
            if let Some(editable) = edit_state.get_node_mut(node_id) {
                editable.modified = true;
            }
        }
    }
}

Supported Syntax

• - [ ] Unchecked task
• - [x] Checked task (lowercase)
• - [X] Checked task (uppercase)
• * [ ] Alternative bullet style (asterisk)

Visual Design

• No bullet marker shown for task items (checkbox replaces bullet)
• 2.0px spacing after checkbox for alignment
• Checkbox state syncs bidirectionally with source

Usage

1. Open a markdown file with task list syntax in rendered or split view
2. Click any checkbox to toggle its state
3. Source markdown updates automatically
4. Raw editor view reflects the change

Edge Cases Handled

• Mixed lists (tasks and regular bullets) render correctly
• Nested task lists preserve indentation
• Malformed syntax falls back to text rendering
• Case-insensitive [x]/[X] handling

Scroll stability

Toggling a checkbox changes one character in the source ([ ] ↔ [x]), which updates content_hash but does not change block boundaries. Without special handling, viewport culling treated that as a full invalidation and ran the bootstrap remeasure pass on the next frame, often producing a different total_height and a visible scroll jump. Checkbox clicks also started the scroll-cooldown window (because pointer.any_down() was treated as “user scrolling”), which suppressed the height fixup that would otherwise compensate.

Fix (v0.3.0):

1. Structure-preserving culling — ViewportCullingState stores block_line_ranges. When ranges match the current AST, cached heights are reused and only content_hash is refreshed. See rendered-view-viewport-culling.md.
2. Scroll input detection — is_active_scroll_input() counts wheel and scrollbar drag only, not checkbox clicks. See sync-scrolling.md § Viewport culling and scroll stability.

Manual test: Open a long task list in rendered view, scroll to the middle, toggle several checkboxes — scroll position should not move.