Fix block cursor glyph visibility when colors collide - #2491
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A hardware block cursor is drawn by the terminal: it fills the cursor cell's background with the theme cursor color (sent via OSC 12) but leaves the glyph in its own foreground color. The primary cursor cell deliberately skips the software REVERSED modifier (it double-inverts inside multiplexers like tmux/zellij), so a glyph whose foreground is close to — or identical with — the cursor color is hard to read or vanishes entirely (e.g. a syntax.keyword token in Dracula, where the keyword and cursor share a color). Always repaint the block-cursor cell's glyph to a high-contrast color (black or white by the cursor color's luminance), independent of the glyph's own color, so the cursor reads like a normal reverse-video block. The recolor is applied after color conversion so the 256-color contrast pass cannot undo it, and only the single cursor cell is touched. Bar/underline cursors leave the glyph visible and are unaffected; an unresolvable cursor color (no OSC 12) is left to the terminal's own cursor. Two existing rendering tests sampled the exact cell the caret sits on (the first ANSI block; line 1's keyword in the CRLF highlight test); move their caret off the sampled cells so they observe content color rather than the cursor glyph. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Ta2tojr71nT75V96rmz9ci
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Summary
Fixes an issue where glyphs under a block cursor become invisible when their foreground color matches the terminal cursor color (e.g., a keyword token in Dracula theme where both share the same magenta color).
Changes
Glyph collision detection and fix: Added
fix_block_cursor_glyph_collision()method that detects when a glyph's foreground exactly matches the cursor color and recolors it to maintain visibilityCursor style classification: Added
is_block()method toCursorStyleto identify which cursor styles fully cover the glyph cell (block cursors) versus those that leave it visible (bar/underline)Theme color override utility: Added
override_theme_colors()method to Editor for test support, allowing runtime color manipulation to reproduce specific collision scenariosComprehensive test coverage:
Implementation Details
The fix avoids using the software
REVERSEDmodifier (which would double-invert in multiplexers like tmux/zellij) and instead performs a targeted foreground inversion only on the colliding glyph. The replacement color selection prioritizes the cell's own background, then falls back to editor background and foreground to ensure the result never collides with the cursor color itself.https://claude.ai/code/session_01Ta2tojr71nT75V96rmz9ci