feat(trace-viewer): show screencast frames during playback

[Pavel Feldman (pavelfeldman)]

Summary

• Show the screencast in the snapshot area while playing and while dragging the scrubber, so that animations are visible.
• Snap back to the action snapshot when the playback stops, the scrubber is released, an action is selected or hovered, or a locator is picked.
• Traces with a video play the video itself, the thumbnails only index the film strip lanes.

[github-actions]

Hi, I'm the Playwright bot and I took a look at the CI failures here.

🟢 CI looks clear — the one failure is a known flake

[firefox] › mcp/cli-core.spec.ts:149 › upload multiple files is bimodal on mcp-windows-latest-firefox — 6 failures in 696 runs, with the same assertion failing on main (sha 52a0e720) and on unrelated PRs #42431, #42628 and #42713. This PR only touches trace-viewer UI code, which the MCP file-upload flow doesn't reach.

Details

Overall: 1 failed, 10 flaky across the two reports. Nothing points at this PR — the diff is confined to packages/trace-viewer/src/ui/* (playbackControl.tsx, snapshotTab.tsx, ariaModeView.tsx, workbench.tsx, snapshotTab.css) plus a new tests/library/trace-viewer-scrub.spec.ts. That new test passed.

Pre-existing flake / infra

• [firefox] › mcp/cli-core.spec.ts:149 › upload multiple files @mcp-windows-latest-firefox — pre-existing flake. Run history for this test on this bot (oldest first, one square per run attempt):

  🟩🟥🟩🟩🟩🟩🟩🟩🟩🟩🟩🟥🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩🟥

  It's Windows-only in practice (6/696 there vs 1/706 on Linux and 1/709 on macOS). The failing assertion is the same one every time — the browser_file_upload tool errors out because no file chooser is showing yet, so the expected await fileChooser.setFiles( never lands in the code output. That's a timing race between the click and Firefox surfacing the chooser, unrelated to anything here.

• The 10 flaky tests in "tests 1" (service workers, third-party cookies, page-set-input-files, codegen upload) all passed on retry and sit in library/page areas the diff doesn't touch.

_{Triaged by the Playwright bot - agent run}

[github-actions]

Test results for "MCP"

Warning

The triggering workflow run ended with status cancelled. Results below may be incomplete — blob reports from cancelled or timed-out shards are missing, so passing/failing counts do not reflect the full test suite.

615 passed, 160 skipped

---

Merge workflow run.

[github-actions]

Test results for "tests 1"

Warning

The triggering workflow run ended with status cancelled. Results below may be incomplete — blob reports from cancelled or timed-out shards are missing, so passing/failing counts do not reflect the full test suite.

1217 passed, 5 skipped

---

Merge workflow run.

[Yury Semikhatsky (yury-s)]

what if video drifts away from the action and the image is different from the snapshot? may be confusing for the user that stopped on an action