Visual Layout Unification Plan

Problem Statement

The codebase has inconsistent handling of visual column calculations across different flows:

1. Rendering/cursor detection uses ViewLine.char_mappings indexed by visual column

   • Zero-width chars (ANSI escapes) have no entries, breaking cursor detection

2. Mouse clicks use char_mappings[visual_col] for O(1) lookup

   • Works for visible chars, but same zero-width issue

3. MoveUp/MoveDown use str_width() and byte_offset_at_visual_column() on raw buffer

   • Doesn't understand ANSI escape sequences (counts [, 2, m as width 1 each!)
   • Doesn't handle tab expansion consistently with rendering

Solution: Unified Visual Layout Module

Core Data Model

// Per-line mappings that support all operations
pub struct LineMappings {
    // Per-CHARACTER (indexed by char position in text)
    // Length == text.chars().count()
    pub char_source_bytes: Vec<Option<usize>>,

    // Per-VISUAL-COLUMN (indexed by visual column)
    // Length == visual width of line
    pub visual_to_char: Vec<usize>,
}

Shared Utilities

// primitives/visual_layout.rs

/// Calculate visual width handling ANSI escapes, tabs, zero-width chars
pub fn visual_width(s: &str, start_col: usize) -> usize;

/// Convert byte offset to visual column (ANSI-aware, tab-aware)
pub fn byte_to_visual_col(s: &str, byte_offset: usize) -> usize;

/// Convert visual column to byte offset (ANSI-aware, tab-aware)
pub fn visual_col_to_byte(s: &str, visual_col: usize) -> usize;

/// Build complete per-char and per-visual-col mappings
pub fn build_line_mappings(text: &str, source_bytes: &[Option<usize>]) -> LineMappings;

O(1) Operations

Note: MoveUp/Down is O(n) but only processes one line at a time, and navigation is infrequent compared to rendering.

Implementation Steps

1. Create primitives/visual_layout.rs

• Move/refactor ANSI-aware width calculation from display_width.rs
• Add build_line_mappings() function
• Add byte_to_visual_col() and visual_col_to_byte() with ANSI/tab support

2. Update ViewLine in view_pipeline.rs

• Replace char_mappings: Vec<Option<usize>> (per visual col)
• With char_source_bytes: Vec<Option<usize>> (per char)
• Add visual_to_char: Vec<usize> (per visual col)
• Keep char_styles but make it per-char instead of per-visual-col

3. Update view_pipeline.rs iterator

• Build new mappings structure during iteration
• One entry in char_source_bytes per character (including zero-width)
• One entry in visual_to_char per visual column

4. Update split_rendering.rs

• Cursor detection: use char_source_bytes[char_index] instead of char_mappings[col_offset]
• Track char_index separately from col_offset during iteration
• push_span_with_map: update to build per-char mappings

5. Update input.rs mouse click handling

• screen_to_buffer_position: use visual_to_char[col] then char_source_bytes[char_idx]
• Update ViewLineMapping struct to use new format

6. Update actions.rs MoveUp/MoveDown

• Replace str_width() with ANSI-aware byte_to_visual_col()
• Replace byte_offset_at_visual_column() with ANSI-aware visual_col_to_byte()

Migration Notes

• The existing display_width.rs functions can remain for simple cases
• New visual_layout.rs handles the complex ANSI/tab cases
• Tests will need updating to reflect new data structures