fix(desktop): markdown pages no longer render invisible in the dark-mode browser

[shivamhwp]

Opening a markdown or plain-text URL in the in-app browser (e.g. usenotra.com/pricing.md) in dark mode showed a blank white page. The text was there, but it was white on white. Light mode was fine.

Plain-text documents declare color-scheme: light dark, so in dark mode Chromium makes their text white. Chromium also paints a dark background for these pages, but only when the frame's base background is opaque. Electron gives <webview> guests a transparent base, so the dark background is skipped and the white backing from #9463 shows through. This affects any page that relies on a dark color scheme without setting its own background.

The fix sets an opaque white base background (Emulation.setDefaultBackgroundColorOverride) on each browser tab when the preview's debugger attaches. Chromium then paints its dark background for dark-scheme pages, as Chrome does. It's sent on every debugger attach, including after a webview swap or after DevTools closes.

What doesn't change

The white matches the white backing the webview already has (#9463), so light pages and pages with their own background look exactly as before. On screen, only dark-scheme pages with no background change: from invisible to readable. One more change: about:blank shows Chromium's dark background in dark mode instead of white. Tested on 11 page types (plain, explicit background, semi-transparent, light dark, dark-only, only light, dark site with its own background, text/plain, image, iframe, about:blank) in light, dark, and the panel's per-tab Dark.

Limits

Chromium applies the override only to documents created after it's set. That leaves three cases:

• Pages with iframes: dark-scheme pages that contain an iframe, even a same-origin one, stay white-on-white. Re-sending the override doesn't help.
• Pages already loaded when the debugger attaches: they stay broken until their next load. That covers tabs restored at app launch, and tabs after DevTools closes.
• Tabs with DevTools open: Chromium shares the override across debugger clients, so DevTools on the tab clears it until the next load. Dev builds auto-open DevTools for the app window, and that attaches to tabs too, so close it before testing in vp run dev:desktop.

None of these is worse than main. They're the same broken render as today.

Side effect: captures now match the screen

Agent snapshots, copied screenshots, annotation crops, picture-in-picture frames and debugger screencast frames capture the tab without the panel's white backing. On main, pages with no background of their own capture as transparent PNGs, and as black in JPEG and screencast frames: black text on black. With this change, every capture matches the screen. Tested: before, 25 of 33 page × theme scenarios captured differently from the screen; after, only the iframe pages still do.

Before (current main), in the T3 desktop app

Before / after

Each clip loads the page in light, then switches the theme to dark while it's open. Both come from a standalone harness: Electron 44.4.2 (the app's version) with a white <webview> like the browser panel's. The only difference between them is this change.

Before	After

Verification

• vp run typecheck in apps/desktop
• vp lint on the changed files
• vp test run apps/desktop/src/preview/Manager.test.ts (93 passed). The extended test fails without the fix.

Made with Claude Opus 5.5 in Claude Code, running in T3 Code.

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR changes the default background rendering applied to every desktop preview webview, affecting light, dark, and system-scheme pages rather than only adding an opt-in path. The behavior is localized and tested, but the default-rendering change and scheme-handling implications warrant human review.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.5 KiB	−67 B (−0.5%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+4 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.4 KiB	−71 B (−1.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.3 KiB	56.2 KiB	−44 B (−0.1%)	66.4 KiB	✅
Codex	Live turn messages	10	9	−1 (−10.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+12 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+3 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+9 B (+0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 5cc99e1 · PR result: b818154 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 114.0 KiB
• Claude decoded thread snapshot: 114.7 KiB

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• apps/desktop/src/preview/Manager.ts

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• apps/desktop/src/preview/Manager.ts

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📝 Walkthrough

Walkthrough

Preview control session initialization now sends Chromium’s CDP command to set the guest’s default background to opaque white. Tests check this setting on initial and replacement webviews.

Changes

Preview control session

Layer / File(s)	Summary
Set and verify default background color apps/desktop/src/preview/Manager.ts, apps/desktop/src/preview/Manager.test.ts	initializeControlSession sends Emulation.setDefaultBackgroundColorOverride with opaque white. Tests check the override on the initial and replacement webviews.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to b8181

The background override enables dark-scheme text pages to receive a dark canvas. The previous white-on-white concern is contradicted by the supported runtime’s implementation; this change is mergeable after normal checks.

Architecture Summary

Architecture risk: 🔵 Low · up to 2a1f8

The change affects 1 system.

Changed systems: apps/desktop

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — apps/desktop (service) was modified; 2 changed files map to changed impact.

Before / after behavior

• observed — Modified behavior in apps/desktop/src/preview/Manager.ts: initializeControlSession now sets the guest’s default background to opaque white through CDP after enabling Runtime, Accessibility, Network, and Log. A command failure propagates as an initialization error.
• observed — Modified behavior in apps/desktop/src/preview/Manager.test.ts: The test description now covers reapplying an opaque base and color scheme across webview swaps, replacing its prior description focused on emulating prefers-color-scheme.
• observed — Modified behavior in apps/desktop/src/preview/Manager.test.ts: The test now expects the initial webview to receive a default background-color override with white RGB values and alpha 1.
• observed — Modified behavior in apps/desktop/src/preview/Manager.test.ts: The test now expects the replacement webview to receive the same opaque white default background override.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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 2…
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.
Title check	✅ Passed	The title is concise, uses the repository's conventional commit format, and clearly identifies the primary desktop dark-mode rendering fix.
Description check	✅ Passed	The description clearly explains the problem, implementation, limitations, side effects, visual evidence, and verification results. It does not include a separate Scope and approval section or explici…

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Inline comments:
Review comments at @apps/desktop/src/preview/Manager.ts:
- Line 1429: Update the fallback color used by setDefaultBackgroundColorOverride
in the Manager preview flow to match the effective color scheme, resolving
system mode through the effective system/document scheme and avoiding an
override that conflicts with an explicitly authored document scheme.

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• apps/desktop/src/preview/Manager.ts

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