terminal: reset style before VT replay of blank cells

[lawrencecchen]

The VT formatter counted unstyled blank cells and wrote them as spaces just before the next non-blank cell, but it wrote them before it switched to that cell's style. The spaces took the previous cell's colors.

Claude Code draws its mascot with ESC[48;2;0;0;0m (black background) and then jumps over three untouched cells with ESC[12G before ESC[49m. A live terminal leaves those cells empty. A cmux Cloud replay (cmux-tui vt_replay through libghostty-vt) painted them as a black box next to the mascot.

The fix closes a non-default style before writing the pending blanks, the same way the row-break path already does. Plain output is unchanged.

Commits: 2439e8e7c adds the failing test Page VT unstyled blank cells do not inherit the previous background, 51c8da0ce adds the fix.

Verification on a Blacksmith Testbox (Linux, Zig 0.16.0): zig build test-lib-vt -Dtest-filter='Page VT' fails at the test commit (AB ESC[0mC) and passes at the fix commit. Full test-lib-vt has one failure, kitty temporary file medium preserves bool ABI, which also fails at base e1b8bf5f4. cmux-tui cargo test passes except one mouse-capture test that passes when run alone.

Upstream ghostty-org/ghostty has the same code; upstreaming is a maintainer decision.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the VT formatter writing blank cells with the previous cell's colors, which could paint a black box in replays after Claude Code's mascot output.

The formatter now closes an open non-default style before emitting accumulated blank spaces, matching the row-break path. Plain output is unchanged, and a regression test covers a styled cell followed by skipped default cells.

^{Written for commit 51c8da0. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Blank spaces between styled text and subsequent content now retain their default appearance instead of inheriting the preceding text’s styling.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: e6b4b17f-791a-403b-bf4c-76b044858ad7

📥 Commits

Reviewing files that changed from the base of the PR and between e30b480 and 51c8da0.

📒 Files selected for processing (1)

• src/terminal/formatter.zig

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📝 Walkthrough

Walkthrough

PageFormatter now resets active non-default styling before emitting accumulated blank cells. A regression test checks that three unstyled spaces follow an SGR reset after text with a black background.

Changes

Blank-cell formatting

Layer / File(s)	Summary
Reset style before blank cells src/terminal/formatter.zig	PageFormatter closes an active non-default style and resets tracked style state before writing accumulated blank cells. A regression test checks that an SGR reset precedes three unstyled spaces.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: mitchellh

Merge Risk: ⚪ Minimal · up to 51c8d

The formatter now emits pending blank cells without inheriting the prior style, and the regression test checks the reset ordering. No actionable merge-blocking risk is evident.

Architecture Summary

Architecture risk: 🔵 Low · up to 51c8d

The change affects 1 system.

Changed systems: src

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — src (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in src/terminal/formatter.zig: Before emitting accumulated blank cells, styled output now closes a non-default style and resets the style state; previously the spaces were emitted without this reset.
• observed — Modified behavior in src/terminal/formatter.zig: Adds a regression test that formats text with a black background, skips cells, resets the background, and writes another character. It expects an SGR reset before the three unstyled spaces so they do not inherit the previous background.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: resetting terminal style before replaying blank cells in VT formatting.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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