[modacker]

Change

The agent-transcript integration case waits for the browser panel's first frame, and when the frame never arrives the failure has to say what the panel was doing. It could not.

The panel has six states, each carrying its own testid (packages/webui/src/client/features/browser/ui/BrowserPanel.public.tsx). The diagnostic looked at two of them -- the empty placeholder and the capability notices -- and both read the same in every failure: the placeholder is on screen whether the stream is slow, whether the stream failed, or whether the host cannot serve a browser at all. The product's own first-frame deadline is 60 seconds (packages/webui/src/runtime/browser/browser-service.ts), the same budget the wait had, so a panel that had already given up used to look exactly like a panel that was merely slow.

The wait now ends on whichever comes first: the frame, or the panel's own failure notice. A failure prints every browser-panel-* node that was on the page, with the browser's error code beside the notice's words, so the next occurrence names the state instead of guessing at it.

This is the readable-failure half of #32. It does not change what the product does, and it does not change what the test asserts: the frame still has to arrive.

Validation

• Checks run and results (include the revision/profile where relevant):
  • pnpm --filter @mavis/webui typecheck:test: pass.
  • Vitest, the whole webui suite: 302 files, 4525 tests: pass.
• Performance: basic -- the automatic 100-round check. No perf:full label: the change is outside the paths the performance rules in CONTRIBUTING.md list, and it is not a performance optimization.
• NOT RUN, platform limitations and live-service boundaries:
  • The case this makes readable fails intermittently (recorded in [Bug]: the agent-transcript case fails on the Windows runner and the failure leaves no evidence behind #32), and this change does not make the product produce a frame it was not producing. It makes the failure say which state the panel was in.
  • The CI-level half of [Bug]: the agent-transcript case fails on the Windows runner and the failure leaves no evidence behind #32 -- the failing step's own diagnostics being removed before the artifact is uploaded -- lives in the workflow layer (.github/workflows/webui-ci.yml and playwright.config.mjs), outside this change, and is still open.
  • The panel's failure notice is read from the page, so it only appears if the product got as far as rendering one. A panel that never rendered anything still fails as a timeout, now with every panel node in the message rather than two guesses.

Publication and contribution checks

• I have permission to contribute these changes under the existing licenses applicable to the changed files/packages; imported material and its provenance are identified and existing notices are preserved.
• No credentials, account data, real user content, internal source history or private review material is included.
• Added/removed source files were reviewed before regenerating release/public-source.json; new tests are declared in test/vitest-suites.json where applicable. -- no source file was added or removed, so release/public-source.json is unchanged; no vitest file was added, so no suite declaration is needed.
• Shared English/Chinese documentation and capability/verification records are updated where applicable. -- no shared documentation describes this diagnostic, and the product's behaviour is unchanged.

Maintainer handoff

Publication scope or license changes (if any): none.

Shared-source port: not needed. The change is inside packages/webui, which does not exist upstream; per packages/webui/docs/agents/issue-tracker.md nothing in this repository is published to upstream.