fix(web): erase terminal cursor fringes between blinks

[Lucenx9]

What changed

Snap terminal row boundaries to device pixels and use those same boundaries for the cursor, backgrounds, selection, and text clipping. Adjacent rows share an edge, so repainting the cursor row does not touch its neighbors.

The production change is confined to the shared web/desktop renderer. Blink timing, product defaults, and the terminal protocol stay unchanged. Mobile uses its separate native renderer.

Why

At fractional display scales, repainting an antialiased row boundary blends over the cursor edge instead of fully erasing it. This leaves faint horizontal lines during the blink's off phase. Pixel-aligned boundaries fix that without repainting the entire terminal on every blink.

Closes #11402.

flowchart LR
    A[Cursor on] --> B[Blink off]
    B --> C[Repaint cursor row]
    C --> D[Shared pixel edges erase the cursor completely]

Loading

UI changes

Before/after Chromium reproduction using the actual renderer from b1e223e2b0 and this branch, with identical input. The PNGs are captured at 2x density; the inset enlarges the original canvas pixels 8x. These are isolated renderer captures, not full-app screenshots.

Before	After

Short before/after video. Full comparison and integrated-terminal evidence.

Verification

• 66 focused tests pass in renderer.test.ts and surface.test.ts. The new geometry tests fail at 125% and 150% on the base renderer.
• Web package typecheck and targeted lint/format checks pass.
• All 97 real-canvas comparisons pass after the fix, covering four cursor styles, light/dark themes, text, selections, adjacent rows, four display scales, and a fractional CSS origin.
• Tested the full app in Chromium with isolated state and synthetic shell output. At 125%, the live cursor leaves no residual pixels, including after scrollback appears.
• Scoped Fallow audit passes with one thread and no introduced findings. Existing findings were left untouched.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Model: GPT-5. Harness: OpenAI Codex.

Summary by CodeRabbit

• Bug Fixes

  • Improved terminal rendering across different display scales by aligning row boundaries to device pixels.
  • Corrected row heights for backgrounds, selections, text, decorations, and cursor rendering to maintain consistent vertical alignment.
  • Limited cursor updates to the affected row when appropriate, improving repaint behavior.

• Tests

  • Added coverage for pixel alignment, contiguous row boundaries, and targeted cursor repainting across multiple display scales.

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-12T13:41:13.315318Z	48311fb	PR opened

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[macroscopeapp]

Approvability

Verdict: Approved at 48311fb

Macroscope's review found this PR approvable — This is a small, self-contained terminal rendering bug fix that aligns row and cursor edges to device pixels to prevent blink artifacts. Production impact is confined to existing canvas rendering, with targeted fractional-scale and cursor repaint tests added.

No code changes detected at ff04249. Prior analysis still applies.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: dd4dd2e5-85bb-415f-bf81-5eae2da592c5

📥 Commits

Reviewing files that changed from the base of the PR and between b1e223e and 48311fb.

📒 Files selected for processing (3)

• apps/web/src/terminal/ghostty/renderer.test.ts
• apps/web/src/terminal/ghostty/renderer.ts
• apps/web/src/terminal/ghostty/surface.test.ts

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📝 Walkthrough

Walkthrough

Ghostty terminal rendering now snaps row boundaries to device pixels. Backgrounds, text clipping, decorations, and cursors use the computed row geometry. Tests cover multiple display scales and canvas transform mocks.

Changes

Terminal rendering

Layer / File(s)	Summary
Transform-aware row geometry and rendering apps/web/src/terminal/ghostty/renderer.ts, apps/web/src/terminal/ghostty/renderer.test.ts, apps/web/src/terminal/ghostty/surface.test.ts	renderGhosttySnapshot snaps row boundaries using the canvas transform. Row backgrounds, selection, text clipping, decorations, and cursors use the computed row height. Tests verify pixel alignment, contiguous rows, repaint behavior, and getTransform() mocks.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: stienswout

Merge Risk: ⚪ Minimal · up to ff042

The terminal rendering change has focused coverage for pixel alignment and blink repaint behavior, with no actionable current-head risk identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the primary fix: removing residual terminal cursor edges during blinking.
Description check	✅ Passed	The description includes the required change summary, rationale, UI evidence, verification details, and completed checklist items. It is focused and aligned with the template.
Linked Issues check	✅ Passed	Issue #11402 requires the cursor to disappear during the blink-off phase without residual cell edges, while preserving the background and text. renderer.ts snaps shared row boundaries with the canva…
Out of Scope Changes check	✅ Passed	The changes stay within Issue #11402. They modify the shared Ghostty web renderer, add focused renderer coverage, and update canvas test doubles with getTransform(). They do not change blink timing,…

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[greatitself]

Independently validated 48311fb in Chromium using the actual renderer from that commit and base b1e223e.

• The 125% scale reproduction leaves 26 residual pixels on the base and zero on this PR after the cursor blinks off.
• 192 canvas comparisons passed across DPRs 1, 1.25, 1.5, and 2; integer and fractional row origins; light/dark, glyph, colored background, selection, and strikethrough cases; and all four cursor styles. Each case runs three blink cycles followed by hiding the cursor with a changed row, then verifies the final canvas exactly matches its original cursor-off pixels.

Blink recording · Evidence and test details

These are isolated renderer captures, not a separate Electron application test.

Model: GPT-6. Harness: Codex.