fix(desktop): markdown no longer loads invisible in new dark-mode browser tabs

[shivamhwp]

Follow-up to #14601. In dark mode, opening a markdown or plain-text URL in a new browser tab could still show a blank white page on its first load. Reloading fixed it. It happened most on the first tab after T3 starts.

#14601 gets Chromium to paint its dark background by setting an opaque base color through the tab's debugger session. That session only opened after the web UI registered the tab. A tab created with its URL already set starts loading right away, and a fast page could paint before the session opened. Chromium keeps the transparent base for a document that has already painted, so the text stayed white on white until the next load. Typing a URL into a fresh tab, clicking a link in chat, agent-opened tabs, and tabs restored at launch all create tabs this way.

The desktop main process now opens the tab's debugger session as soon as Electron attaches the tab, and sends the opaque base before the other setup commands. Registering the tab reuses that session. Light pages look the same as before.

Before (main)

The first load is white on white. A reload of the same page renders it readable.

After (this branch)

Testing

I ran vp run dev:desktop under Xvfb with fresh worktree state and the dark theme, and served a page as text/plain from localhost. Each run opened the Browser panel and typed the page's URL into the new tab. I read the rendered pixel from the X display.

• On main, the first tab after the app started was white on white in 3 of 5 cold starts. New tabs in an app that was already running rendered correctly 5 of 5 times, so this is a race that a cold start usually loses.
• On this branch, the first load was readable in 4 of 4 cold starts and 10 of 10 new tabs in a running app.

I also checked these on this branch:

• A new tab in light mode still renders a white page with dark text.
• The tab's Appearance menu set to Dark or System in a light app switches the page live, and so does switching the app theme.
• Agent automation (status, snapshot, evaluate) works through the session opened at attach.
• Opening DevTools on the tab still blocks automation, and closing DevTools restores it.

I did not separately test tabs restored after a window reload, because tabs on a draft thread don't come back in my test setup. Those tabs mount through the same path as the new tabs above.

vp test run src/preview/Manager.test.ts passes (94 tests). The new test fails if the opaque base is not the first command after the debugger attaches.

Dev builds open DevTools for the app window automatically, and that DevTools attaches to browser tabs and clears the override, as noted in #14601. I turned that off locally while testing.

Pages with iframes and tabs with DevTools open behave as they did after #14601.

Made with Claude Opus 5.5 in Claude Code.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This targeted fix moves debugger initialization onto the webview-attach path so dark-mode documents receive their background before first paint. Because initialization awaits pre-navigation debugger commands while holding a shared session lock, a pending command could stall later tabs and destruction cleanup; that production availability risk needs human review.

No code changes detected at 5b5b627. Prior analysis still applies.

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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	4.9 KiB	4.9 KiB	−40 B (−0.8%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.7 KiB	3.7 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.1 KiB	−40 B (−3.3%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.4 KiB	20.4 KiB	−41 B (−0.2%)	29.3 KiB	✅
Codex	Live turn messages	2	1	−1 (−50.0%)	8	✅
Claude	Total thread wire	4.9 KiB	4.9 KiB	+41 B (+0.8%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.7 KiB	3.7 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	+41 B (+3.5%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	20.7 KiB	20.8 KiB	+41 B (+0.2%)	29.3 KiB	✅
Claude	Live turn messages	1	2	+1 (+100.0%)	8	✅

Baseline: fd7ee2c · PR result: 5b5b627 · 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: 106.1 KiB
• Claude decoded thread snapshot: 106.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

Dismissing prior approval to re-evaluate 12a1df2

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• apps/desktop/src/preview/Manager.test.ts
• apps/desktop/src/preview/Manager.ts

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📝 Walkthrough

Walkthrough

DesktopWindow starts debugger preparation after it attaches a webview. PreviewManager opens a control session, applies the opaque white background override before enabling debugger domains, and detaches the session when the guest is destroyed.

Changes

Webview preparation

Layer / File(s)	Summary
Initialize and clean up debugger sessions apps/desktop/src/preview/Manager.ts, apps/desktop/src/preview/Manager.test.ts	PreviewManager checks whether a guest is still alive before opening a control session. It applies the default background override before enabling debugger domains. Tests cover session reuse, guest destruction, and concurrent preparation.
Invoke preparation after attachment apps/desktop/src/preview/Manager.ts, apps/desktop/src/window/DesktopWindow.ts, apps/desktop/src/window/DesktopWindow.test.ts	DesktopWindow invokes prepareWebview after attaching a webview. PreviewManager exposes and wires the operation. The window test mock provides a no-op implementation.

Priority: ➖ Normal

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant DesktopWindow
  participant PreviewManager
  participant WebContentsDebugger
  DesktopWindow->>PreviewManager: prepareWebview(contents)
  PreviewManager->>WebContentsDebugger: open control session
  PreviewManager->>WebContentsDebugger: set opaque white background override
  PreviewManager->>WebContentsDebugger: enable Runtime, Accessibility, Network, and Log
  WebContentsDebugger-->>PreviewManager: destroyed event
  PreviewManager->>WebContentsDebugger: detach session

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Suggested reviewers: juliusmarminge

Merge Risk: 🟡 Moderate · up to d53d5

A stalled debugger command could prevent later tabs from preparing and delay session cleanup. Resolve the lock-held wait before merging, or explicitly accept that risk.

Security Architecture Review

Security architecture risk: 🔵 Low · up to d53d5

Existing tab isolation checks remain in place. However, a stalled tab setup could delay other tabs and their cleanup. The change broadens this potential failure mode, although no practical exploit or privilege bypass was established.

Retained concerns

• Medium · reliability · inferred: Attach-time preparation broadens a pre-existing shared-lock hazard to unregistered guests. If a debugger initialization command remains pending, it holds the session-map synchronization lock, delaying other guests’ initialization and destruction cleanup. The destroyed check handles queued guests only after lock acquisition; it does not interrupt the current initializer. Production command settlement remains unverified.

Security review details

Security Blast Radius

• inferred — The identified failure-containment exposure spans guests sharing one preview manager’s session map, including unclaimed guests. It does not establish access to another tenant, service, environment, or data store, and attacker ability to force a persistent debugger-command stall remains unproven.

Trust Boundaries and Controls

• observed — The examined attachment path passes Electron-provided guest identity, not a renderer-supplied identifier. Renderer registration still validates ownership before publishing the guest into tab state, and later WebContents lookup resolves through committed tab state. No arbitrary guest-selection bypass was established for the new method.

Resilience and Maintainability Implications

• observed — Session cleanup removes diagnostic state and debugger listeners and attempts detachment. However, destruction first obtains the same synchronized session reference used by initialization; it is not an independent cancellation path for an in-flight initializer.

Hardening Proposals

• proposed — Consider per-guest initialization ownership with bounded command waits and destruction-triggered interruption, keeping shared-map publication short and preserving session reuse. This would contain stalled initialization without depending on undocumented native settlement behavior.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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 clearly identifies the desktop fix for invisible markdown content in new dark-mode browser tabs.
Description check	✅ Passed	The description explains the problem, implementation, scope, verification steps, observed results, screenshots, untested restored tabs, and agent attribution. It does not use the template headings or …

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Actionable comments posted: 1

🧹 Nitpick comments (1)

apps/desktop/src/preview/Manager.test.ts (1)

1750-1751: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Test destruction before tab ownership.

The test registers guest 44 before invoking its destroyed callback. The callback therefore runs after the tab owns the guest, not in the unclaimed-guest state. Prepare a separate guest and invoke its callback before any registerWebview call. Keep guest 44 for the ownership and reuse assertions.

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Review comment at @apps/desktop/src/preview/Manager.test.ts around lines 1750 -
1751:
Update the destruction test to use a separate guest whose destroyed callback
runs before any registerWebview call, exercising the unclaimed-guest path. Keep
guest 44 for the existing ownership and reuse assertions.

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Inline comments:
Review comments at @apps/desktop/src/preview/Manager.ts:
- Line 2432: Update ensureControlSession so controlSessionsRef protects only
reserving the session, then run debugger-command initialization outside the
lock; make initialization cancellable when the webContents is destroyed so
cleanup can detach the session even if a command remains pending.

---

Nitpick comments:
Review comments at @apps/desktop/src/preview/Manager.test.ts:
- Around line 1750-1751: Update the destruction test to use a separate guest
whose destroyed callback runs before any registerWebview call, exercising the
unclaimed-guest path. Keep guest 44 for the existing ownership and reuse
assertions.

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• apps/desktop/src/preview/Manager.test.ts
• apps/desktop/src/preview/Manager.ts
• apps/desktop/src/window/DesktopWindow.test.ts
• apps/desktop/src/window/DesktopWindow.ts

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