fix(chat): stabilize live transcript activity

[AlphaCatMeow]

Linked issue

Closes #349

Summary

Stabilizes the live assistant activity region so reasoning, tools, results, and the Vibing status update without remounting or reordering already visible transcript content. Bottom-follow writes are coalesced under one owner, detached readers retain their anchor, and thinking details move to a non-layout overlay.

This is a focused GUI/Gateway WebUI fix. It does not change Rust, Go, protocols, databases, or the TodoWrite task-progress feature.

Rebase and diff audit

• rebased the complete two-commit PR from original merge base 849daf269762846557cc8243c5fe6e7fb155ed72 onto latest upstream/main@00a2c6fc43754f40022b0703459824559bee73ea
• updated head from babfd48d0ec8b7cbdaec8db7d1df24116d456849 to f8db66e4f896008bbea23446524e15e2bcf7b724; the branch is 0 behind / 2 ahead and its merge base is the latest main commit
• retained the original two-commit order: live transcript stabilization first, runtime acceptance evidence second; the final amend changes only the worklog while the accepted product tree is byte/path-equivalent
• the product patch remains 44 files; original stats were 1681 insertions / 417 deletions, final stats are 1695 insertions / 417 deletions because the worklog now records the current baseline and fresh acceptance
• only GatewayApp.tsx and scripts/mirror-manifest.json overlap changes made between the old and new baselines; latest-main CLI identity removal and overlay-host placement remain intact together with the PR's viewport-following prop and mirror entries
• pairwise overlap is 6 files with PR feat(chat): add task progress indicator and stabilize ingress replay #345 and 12 files with the live PR feat(chat): add collapsible process details #366 head; neither relationship has Git ancestry, so all three PRs remain independent roots
• no rebase conflicts, Rust/Go/database/protocol/dependency changes, generated binaries, credentials, or unrelated lifecycle changes

Change scope

• Modules: agent-gui, agent-gateway/web
• Stable desktop assistantActivity outer row and live-to-settled identity projection
• Stable ordinary-tool grouping and in-place result/status updates in both clients
• Mirrored thinking overlay, frame-pinned scroll controller, and scroll-adjust policy
• Single-line, width-bounded live status and removal of the duplicate Markdown stream caret
• Projection, component-source, stress, reconnect, special-tool, scroll, accessibility, and layout regression tests
• Mirror manifest and implementation/acceptance worklog

Screenshots / preview

Runtime acceptance was performed from the current main workspace and rebased product HEAD in the Windows Tauri client. The same mirrored GUI/WebUI implementation was covered by focused and full automated suites.

Verification

• pnpm --dir crates/agent-gui build — passed (includes TypeScript typecheck)
• pnpm --dir crates/agent-gateway/web build — passed (includes TypeScript typecheck)
• focused GUI activity/identity/scroll/overlay suites — 46/46 passed
• focused Gateway WebUI activity/identity/scroll/overlay suites — 47/47 passed
• pnpm --dir crates/agent-gui test:frontend — 1,411/1,416 passed; the same five mention source-extraction and Rust/TypeScript preset-sync failures reproduce on a fresh non-Worktree upstream/main@00a2c6fc baseline at 1,398/1,403
• pnpm --dir crates/agent-gateway/web test — 504/504 passed
• full GUI Biome — existing baseline debt only: 424 errors / 358 warnings / 9 infos, identical to the latest-main baseline
• full Gateway WebUI Biome — existing baseline debt only: 292 errors / 310 warnings / 10 infos, identical to the latest-main baseline
• targeted Biome for all 14 touched GUI production files — exited 0 with warnings only
• targeted Biome for all 11 touched Gateway WebUI production files — exited 0 with warnings only
• go -C crates/agent-gateway test ./... — every package passed except the existing Windows file-mode assertion in internal/auth/agenttoken.TestDBFilePermissionsAndNoPlaintext (0666 versus POSIX 0600), reproduced on latest main
• go -C crates/agent-gateway vet ./... — passed
• cargo check --manifest-path crates/agent-gui/src-tauri/Cargo.toml --tests — passed using the existing local Python clang runtime; five unchanged unused/dead-code warnings remain
• node scripts/check-mirror.mjs — passed for 118 mirrored files
• git diff --check — passed
• independent Git, GUI, and Gateway WebUI semantic reviews — no P0/P1/P2 blocker; the details overlay is intentionally user-activated while the live thinkingOpen state drives only the compact running indicator, matching Issue fix: stabilize live transcript activity and scrolling #349 and the acceptance matrix
• GitHub final-head checks for f8db66e4: PR Governance (two runs), Gateway, Gateway Docker Smoke, Gateway WebUI, GUI, Tauri Rust Check, GUI/WebUI Mirror Check, and Diff Hygiene all completed successfully

Manual acceptance

• current-workspace Tauri runtime was launched from chore-pr-350-rebase@e61a2bdf before the documentation-only final amend; Vite returned HTTP 200 and the worktree-local target/debug/liveagent.exe window remained responsive
• the user explicitly confirmed PR #350 通过 on 2026-08-06 after testing stable streaming identity/order, bottom-follow and detached-reader anchoring, thinking overlay interactions, parallel/out-of-order tools, failure/retry, Stop/cancel, AskUserQuestion, history/reconnect restoration, narrow layout, themes, and reduced motion
• the accepted product HEAD and final pushed HEAD differ only in docs/worklog/live-transcript-jitter.md; a path-by-path comparison found zero product-code changes

Pre-submit checklist

• A requirement issue is linked.
• No merge conflict with the target branch after the latest upstream/main fetch and rebase.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for the behavior change and acceptance evidence.

Depends-On: none
Stack-Root: #350

[github-actions]

PR governance checks passed. Awaiting human review.