Fix frontend stream-state, save/continue, chart redraw, and tooltip bugs

[anth-volk]

Continues #164, which was auto-closed by renaming its head branch (GitHub closes a PR when its head branch is renamed; the shorter name keeps the Modal preview hostname under the 63-char DNS label limit — see #180 discussion). Same commits.

Fixes #155
Fixes #180

Single PR fixing the batch of verified frontend bugs in frontend/. Numbering matches the issue.

Changes

High

1. Stream-state corruption when switching/clearing/deleting chats mid-stream (ChatPage.tsx): added a streamGeneration ref. sendMessage and continueMessage capture the generation at start and every state write (updateMessage/flushTarget, the drain interval, 402/429 notices, done handling incl. sessionId.current, saveConversation calls, abort/error handling, and the finally isStreaming/isWaiting resets) bails when stale. startNewChat, loadConversation, and deleteConversation (of the active conversation) call a shared invalidateStream() that bumps the generation, aborts the in-flight request, and resets streaming UI state. saveConversation also gen-guards its late setActiveConversationId so a save completing after a switch can't steal the active highlight. finally blocks only clear abortRef when it still points at their own controller, so a newer stream's Stop button keeps working.

Medium

2. Proxy header drops (route.ts): forwards X-User-Id (with Content-Type) to the backend, and propagates Retry-After on error responses so the frontend's 429 wait message is accurate through the proxy. Superseded by Frontend: forward X-User-Id through proxy, pass through backend errors, fix Continue #146, which also forwards Authorization and passes the backend's real error body through; dropped from this PR during rebase.
3. Floating pipeTo promise (route.ts): .catch(() => {}) on the SSE pipe so client aborts (Stop button) no longer produce unhandled rejections. Superseded by Frontend: forward X-User-Id through proxy, pass through backend errors, fix Continue #146; dropped from this PR during rebase.
4. continueMessage stuck-incomplete on early returns (ChatPage.tsx): snapshots isComplete/stop_reason/stopped before the optimistic clear and restores them on the 402 and 429 branches and on fetch failure, so Copy/cost/Continue survive a failed continuation attempt.
5. sendMessage save omitted turn metadata (ChatPage.tsx): cost_gbp/stop_reason are hoisted out of the done branch and persisted in both the done save and the later suggestions save, matching what continueMessage already saved.
6. Double title generation per turn (ChatPage.tsx): a conversationTitleRef (known title) plus a memoized in-flight generation promise mean chat/title is POSTed at most once per conversation; the suggestions-arrival save reuses the same title. Loaded conversations seed the ref from their existing title, so continuing an old conversation never regenerates it.
7. Auto-scroll no-op (ChatPage.tsx): the old scrollRef container was not scrollable (the document scrolls), so scrollTo did nothing. Replaced with a bottom-sentinel scrollIntoView, gated on already being within 200px of the document bottom so it never fights a user who scrolled up mid-stream; the on-done smooth-scroll trigger is preserved with the same gate.
8. Chart redraw churn (LineChart.tsx, BarChart.tsx, ScatterChart.tsx): scales, domains, margins, categories, and stacked data are now useMemoized, so the svg.selectAll("*").remove() draw effects only re-run when the spec/dimensions actually change instead of on every tooltip-mousemove-driven render.

Low

9. Retry-After NaN (ChatPage.tsx): shared parseRetryAfterSeconds helper guards with Number.isFinite (HTTP-date headers fall back to 60s); used by both send and continue paths.

10. Stop before first token (ChatPage.tsx): the abort path now removes the empty assistant bubble (no text, no tools) instead of marking it stopped with a dead Continue button.

11. Deleting the active conversation (ChatPage.tsx): also aborts any stream, clears messages, sessionId.current, collapsed-working state, and the title refs, and drops the cache entry, so the next send starts fresh instead of resurrecting the deleted conversation.

12. Shared page [CHART_LOADING] (s/[token]/page.tsx): the segment regex now matches [CHART_LOADING] like ChatPage's, so the literal marker no longer renders. Since a saved share can never finish generating the chart, it renders a static "Chart unavailable" placeholder box rather than ChatPage's live spinner.

13. Tooltip dead space below the composer (ChatPage.tsx, fixes Invisible tooltip pseudo-elements add dead scroll space below the composer #180): the data-tip tooltips were hidden with opacity: 0, so their boxes were always laid out and the ones hanging below the composer’s bottom button row stretched the document’s scroll height once the feed exceeded 100dvh. Tooltips now generate no box until :hover (content: none → content: attr(...), fade via keyframe animation), and the attach/Charts tooltips open to the button’s left/right instead of below.

Self-review

Reviewed the full git diff main once after implementing; findings:

• Medium (fixed): loadConversation had no staleness check after its own await, so two rapid sidebar clicks on uncached conversations could apply the older response after the newer one (and the error path could inject an error bubble into the wrong view). Fixed by capturing the generation after invalidateStream() and bailing (success and error paths) if it moved.
• Low (recorded, not addressed — pre-existing or intentional):
  • 402 responses routed through the proxy display the wrapped "Backend error: 402" text rather than the backend's friendly copy (pre-existing: the proxy re-wraps error bodies). Fixed on main by Frontend: forward X-User-Id through proxy, pass through backend errors, fix Continue #146, which passes the backend's error body through unwrapped.
  • continueMessage does not re-save after a late suggestions event, unlike sendMessage (pre-existing asymmetry; out of scope per the no-refactor constraint).
  • The continuation-failure error text is appended to content, which is invisible when the message has events (renderer prefers events; pre-existing). The restored flags make the failure recoverable regardless.
  • Behavior change: loading a conversation now scrolls to its latest message (previously nothing scrolled because auto-scroll was a no-op). This matches typical chat UX.
  • Edge: a user who focuses the textarea, scrolls far up, and presses Enter won't be auto-scrolled to their new message (near-bottom gate). Accepted per the "don't be aggressive" constraint.
  • Streaming auto-follow uses instant scrollIntoView rather than smooth to avoid smooth-scroll churn at the 20ms drain cadence; the on-done scroll stays smooth.

Verification

There is no frontend unit-test suite; verification was npm run build (passes: compile + lint + type-check) plus end-to-end re-reads of each changed flow (send, continue, stop, switch/new/delete mid-stream, save/title, proxy error paths, chart render/tooltip cycles).

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Project	Deployment	Actions	Updated (UTC)
policyengine-uk-chat	Ready	Preview, Comment	Jul 6, 2026 4:02pm

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Beta preview has been cleaned up because this PR was closed.