[Wosca]

Problem

preview_snapshot times out on any desktop preview tab that isn't on screen: tabs opened with open:false, and tabs belonging to a thread that isn't visible, including delegated child threads (#23936). JavaScript, clicks and navigation still work in those tabs. Showing the tab makes the next snapshot succeed.

Inactive preview webviews are moved offscreen to (-100000, -100000). Electron stops compositing a guest that lies fully outside the window, even with background throttling off (see apps/web/src/browser/hostedBrowserWebviewStyle.ts). Every snapshot sends a CDP Page.captureScreenshot, which then waits for a frame that never arrives. Recording and picture-in-picture already avoid this with a background-activity lease. Agent snapshots never took one.

Change

Before sending a screenshot, DesktopBrowserHost asks the window that hosts the tab to make it renderable, and waits for an answer:

• The renderer (browserCaptureSurface.ts) finds the webview by its webContents id and takes the existing acquireBrowserSurfaceActivity lease for that tab only. It answers once two animation frames have passed, so Electron has the new guest bounds before CDP asks for a copy.
• The lease is released when the capture finishes or fails. It is also released if the surface doesn't become ready within 1 s, and after 10 s in case the server gives up on the screenshot. A server timeout can't cancel Chromium's copy request.
• If the tab navigated or its relay closed while waiting, the screenshot is dropped. If the surface never becomes ready, for example because the T3 window is minimized and not painting, the screenshot is still sent, exactly as before this change. A minimized window's on-screen tab could be captured before this change, and still can.
• Screenshots on a tab are still sent one at a time. Leases are counted per capture, so a capture overlapping a recording releases only its own lease.

packages/contracts/src/ipc.ts adds the name of the renderer callback, next to the existing recording trigger.

Scope and approval

This is the main failure reported in #23936. That issue has no maintainer triage label yet. The timeout error that misreported 15,000 ms is fixed separately in #24130. That PR changes only the message, and this PR doesn't depend on it.

Verification

• apps/desktop/src/preview/DesktopBrowserHost.test.ts: four new tests cover waiting for the surface before sending the screenshot, releasing it after success and failure, skipping a screenshot superseded by navigation, and failing at once when no surface is available. All four fail on main. Two more check that a screenshot is still sent when the surface never becomes ready or can't be reached; both fail without the fallback. Existing tests now also wait for the surface before sending.
• apps/web/src/browser/browserCaptureSurface.test.ts: covers leasing only the captured tab while keeping another consumer's lease, releasing when the window never paints, and releasing an abandoned capture and pending work on teardown.
• vp run check:changed: format, lint, knip, typecheck (29 packages) and the 3 changed test files (16 tests) pass.

Manual check in the desktop app (vp run dev:desktop from this branch, macOS), repeating #23936's steps on https://todomvc.com/examples/react/dist/:

Case	Before (installed nightly)	After (this branch)
Parent thread, open:false tab, 3 snapshots	timed out after ~10 s (as reported in #23936)	3 of 3 succeeded
Delegated child (delegate_task), its own open:false tab, 3 snapshots	timed out	3 of 3 succeeded
On-screen tab, T3 window minimized	succeeded	first commit: failed fast with "render surface did not become ready"; with the fallback: succeeded

The open:false and child-thread rows were run on the first commit. The fallback only changes what happens when the surface never becomes ready, which the unit tests cover.

Implemented with GPT-6.1 Sol (gpt-6.1-sol) in the Codex harness; PR prepared by Claude Opus 5.5 (claude-opus-5-5) in the Claude Code harness, both run from T3 Code.

[Wosca]

Before fixes:

First fix with regression:

Final fix working:

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📝 Walkthrough

Screenshot capture now requests a temporary renderable surface in the browser renderer. The renderer tracks capture activity and paint readiness. The desktop host proceeds after readiness, timeout, or setup failure, and requests surface deactivation without waiting.

Changes: Desktop preview capture

Layer / File(s)	Summary
Install and manage the capture surface packages/contracts/src/ipc.ts, apps/web/src/browser/browserCaptureSurface.ts, apps/web/src/browser/ElectronBrowserHost.tsx, apps/web/src/browser/browserCaptureSurface.test.ts	The renderer registers the capture trigger in Electron. It finds the matching preview webview, acquires an activity lease, waits for two animation frames, and releases activity on timeout, deactivation, or cleanup. Tests cover activity counts, readiness timeout, lease expiry, and teardown.
Wrap desktop screenshot capture apps/desktop/src/preview/DesktopBrowserHost.ts, apps/desktop/src/preview/DesktopBrowserHost.test.ts	The desktop host requests surface activation before capture and requests deactivation afterward. It checks whether the screenshot turn remains valid and captures directly when the host or guest is unavailable. Tests cover readiness outcomes, navigation, timeouts, restoration, and screenshot ordering.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant DesktopBrowserHost
  participant ElectronBrowserHost
  participant browserCaptureSurface
  participant PreviewWebview
  DesktopBrowserHost->>ElectronBrowserHost: Request capture surface activation
  ElectronBrowserHost->>browserCaptureSurface: Dispatch capture trigger
  browserCaptureSurface->>PreviewWebview: Find matching webview and wait for two animation frames
  PreviewWebview-->>browserCaptureSurface: Paint readiness
  browserCaptureSurface-->>DesktopBrowserHost: Return activation result
  DesktopBrowserHost->>DesktopBrowserHost: Capture screenshot if the turn remains valid
  DesktopBrowserHost->>ElectronBrowserHost: Request capture surface deactivation
  ElectronBrowserHost->>browserCaptureSurface: Dispatch capture trigger

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Possibly related PRs

• pingdotgg/t3code#24056: This PR adds capture-surface preparation to the screenshot queue and turn checks introduced there.
• pingdotgg/t3code#24130: Both PRs address the preview snapshot operation; this PR changes capture behavior, while that PR changes timeout handling and diagnostics.

Suggested labels: bugfix

Suggested reviewers: shivamhwp

Merge Risk: ⚪ Minimal · up to f5d52

The change is mergeable after normal checks; no unresolved capture failure is established.

Pre-merge checks: 3 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Description check		The description clearly documents the problem, implementation, verification, and manual results. However, the Scope and approval section does not provide explicit maintainer approval or explain why th…	Add a link to the triaged issue or maintainer approval comment. If using the focused-fix exemption, explain why this obvious bug fix qualifies. Embed or link before/after screenshots and a short recording if timing or interaction details re…

Passed checks: 3 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: preventing timeouts when capturing screenshots from background preview tabs.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description clearly documents the problem, implementation, verification, and manual results. However, the Scope and approval section does not provide explicit maintainer approval or explain why the change qualifies for the approval exemption. The UI verification also does not embed or link the required before/after screenshots or recording.

Resolution

Add a link to the triaged issue or maintainer approval comment. If using the focused-fix exemption, explain why this obvious bug fix qualifies. Embed or link before/after screenshots and a short recording if timing or interaction details require visual evidence.

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Inline comments:
Review comments at @apps/desktop/src/preview/DesktopBrowserHost.ts:
- Line 152: Update withCaptureSurface so its finally block starts
setSurface(false) without awaiting it, allowing the captured screenshot result
and screenshotTurn queue to proceed independently of renderer response. Also
bound the setSurface(true) activation wait so an unresponsive renderer cannot
block capture indefinitely; use the existing captureId lease timeout where
available.

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📒 Files selected for processing: 6

• apps/desktop/src/preview/DesktopBrowserHost.test.ts
• apps/desktop/src/preview/DesktopBrowserHost.ts
• apps/web/src/browser/ElectronBrowserHost.tsx
• apps/web/src/browser/browserCaptureSurface.test.ts
• apps/web/src/browser/browserCaptureSurface.ts
• packages/contracts/src/ipc.ts

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[artjom-mashanin]

Note

Posted by Claude Code (Opus 5.5) on behalf of the account owner.

From reading the diff (not run): the lease is taken only around Page.captureScreenshot. That fixes agent snapshots. It probably doesn't fix remote viewers of a background tab, i.e. the "Connecting to browser…" case in #23015 / #23139.

attachViewer calls Page.startScreencast and then pushStill (apps/server/src/preview/ServerBrowser.ts:2408-2410). With this PR, I expect the following:

• pushStill succeeds, so the remote pane leaves "Connecting…". That's an improvement.
• The lease is released straight after that capture, and the guest returns to (-100000, -100000). The screencast then stops producing frames, so the viewer sees a frozen picture. A frame would arrive only when another screenshot happened to take a lease.

This is the main case for people driving a host remotely (phone, or a second desktop through T3 Connect): the thread usually isn't open on the host, and since #19919 its tabs are still desktop-hosted.

Suggestion: hold one acquireBrowserSurfaceActivity lease per tab while it has at least one viewer attached. Take it in attachViewer and release it in that scope's finalizer. Then both the screencast and the first still render. A timeout on pushStill in attachViewer would also stop one lost capture from leaving the pane on "Connecting…" forever.

Environment where I traced this: the host is the macOS desktop app on 0.1.3-nightly.20261023.3579 (it runs the server itself, no background service), viewed from the macOS desktop app on a second Mac via T3 Connect.

[Wosca]

Moving this back to draft. #22743 fixes the same problem more completely. Its Electron table shows that bringing a parked <webview> back into the window for capture, which is what this PR does, never gets a fresh frame while T3 is hidden, and only sometimes while it is minimized. A tab created while the window is hidden can't be fixed this way at all. #22743 also moves agent tabs out of <webview>s, so this lease would be dead code once it lands.

On the remote-viewer point above: agreed. A capture-scoped lease gets the first still to a remote viewer, but the screencast freezes afterwards. A viewer-scoped lease would be a separate fix, and #22743's hidden BrowserWindow plus the server stream view should cover that case too.

I've posted the manual results and screenshots from this branch on #22743, and will close this if that one lands.