Galley-Based Cursor Positioning

Overview

Implemented pixel-accurate cursor positioning when clicking on formatted text (bold, italic, code, etc.) in WYSIWYG mode. Uses egui's Galley text layout system instead of proportional width estimation.

Problem

When clicking on rendered markdown text containing formatting like **bold** or *italic*:

• The displayed text has different length than raw text (markers stripped)
• Character widths vary based on font metrics
• Simple proportional mapping (click_x / width * text_len) produced inaccurate cursor positions

Solution

Use egui's Galley::cursor_from_pos() for font-metric-aware character index mapping:

fn compute_displayed_cursor_index(
    ui: &Ui,
    displayed_text: &str,
    click_pos: egui::Pos2,
    text_rect: egui::Rect,
    font_size: f32,
    editor_font: &EditorFont,
) -> usize {
    // Create galley with same font as rendering
    let font_family = fonts::get_styled_font_family(false, false, editor_font);
    let font_id = FontId::new(font_size, font_family);
    
    let galley = ui.fonts(|f| {
        f.layout_no_wrap(displayed_text.to_owned(), font_id, Color32::PLACEHOLDER)
    });

    // Convert click to local coordinates
    let local_pos = egui::Vec2::new(
        click_pos.x - text_rect.min.x,
        click_pos.y - text_rect.min.y,
    );

    // Get exact character index
    let cursor = galley.cursor_from_pos(local_pos);
    cursor.ccursor.index
}

Key Files

File	Purpose
src/markdown/editor.rs	compute_displayed_cursor_index, click handlers
src/markdown/rendered_session.rs	PendingActivation applied on formatted block open

Session entry points

Formatted display clicks call session.switch_to_ui(block, PendingActivation { cursor_char_index, request_focus: true }) — galley mapping runs before switch; the session applies focus and CCursor on the next frame. See rendered-edit-session-formatted.md and rendered-edit-session.md.

Implementation Details

Helper Function Location

compute_displayed_cursor_index() at ~line 2418 in editor.rs

Click Handlers Updated

Three locations now use the Galley-based positioning:

1. render_paragraph_with_structural_keys() (~line 1545)
2. render_paragraph() (~line 2262)
3. render_list_item() (~line 3216)

Font Consistency

• Uses the same base font (get_styled_font_family(false, false, editor_font)) as text rendering
• Font size matches the font_size parameter passed to render functions
• Color is PLACEHOLDER since it doesn't affect measurement

API Reference

egui Galley Methods Used

// Create galley for text measurement
ui.fonts(|f| f.layout_no_wrap(text, font_id, color)) -> Arc<Galley>

// Map position to cursor
galley.cursor_from_pos(local_pos: Vec2) -> Cursor

// Get character index
cursor.ccursor.index -> usize

Limitations

• Uses base font (non-bold, non-italic) for measurement
• Mixed-style text may have slight positioning variance
• Assumes single-line text (no wrapping)

Related Tasks

• Task 4: Maps displayed index → raw index (handles **, *, etc. markers)
• Task 5: Integration testing across all render locations

Version

Introduced in v0.2.5.1 as part of cursor positioning hotfix.