fix: harden thinking display streaming edge cases

[nesquena-hermes]

Summary

This PR is a standalone follow-up to #181.

It hardens the thinking/reasoning display path in the chat UI by:

• hiding partial <think> / <|channel>thought prefixes during streaming so users do not briefly see token fragments
• renaming the local streaming buffer variable for clarity

Reference

• Original feature PR: feat: add support for displaying thinking/reasoning blocks in chat #181

Test plan

• pytest tests/ -x -q (499 passed)

[nesquena-hermes]

Agent review — APPROVED ✅

Reviewed by Hermes agent. Full diff, security audit, test suite, QA harness, and browser feature test all completed.

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Summary: Hardens the thinking/reasoning display path by suppressing partial <think>/<|channel>thought token fragments during streaming, and adds inline think-tag parsing for stored messages in renderMessages() so DeepSeek-style and Gemma 4-style reasoning tokens surface as collapsible cards without a separate structured content array.

Diff: 2 files, +46/-1

• static/messages.js +30/-1: new _thinkPairs constant, _streamDisplay() function, updated _scheduleRender to use it
• static/ui.js +16/-0: think-tag regex parsing inside renderMessages() for <think> and <|channel>thought formats

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Test results:

Suite	Result
pytest tests/ (499 tests)	✅ 499 passed, 0 failed
qa/ harness (21 tests)	✅ 20 passed, 1 skipped (unrelated)
Browser API sanity (11 checks)	✅ all passed

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Security audit: CLEAN

• No eval/exec/atob/document.write patterns
• No external URLs introduced
• innerHTML in _scheduleRender: txt goes through renderMd() (existing safe pipeline); isThinking branch uses a hardcoded literal with no user input — safe
• thinkingText extracted from regex and rendered via esc(thinkingText) in existing thinkRow.innerHTML — safe
• No esc()-in-onclick XSS patterns introduced
• All pre-existing security controls intact: relative_to (6×), Invalid category (1×), inlineMd (7×), SRI hashes (3×)

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Logic correctness: _streamDisplay()

All 12 edge cases verified:

Input	Expected output	Result
<thi (partial tag)	'' (hidden)	✅
<think (partial open)	'' (hidden)	✅
<think>reasoning... (open, no close)	'' + "Thinking…" placeholder	✅
<think>reasoning</think>answer	answer	✅
<|channel>thought\n...<channel|>answer	answer (Gemma 4)	✅
Normal text	pass-through	✅
<b>bold</b> (non-think HTML)	pass-through	✅

renderMessages() think-tag parsing: regex anchored to ^, non-greedy, correctly handles multiline reasoning. Unclosed tags don't match. Only the first matching format is used (guarded by if(!thinkingText)). Both DeepSeek <think> and Gemma 4 <|channel>thought formats extracted correctly.

One design-scope note (not a bug): _streamDisplay() only handles a think block at position 0 of the accumulated buffer. A hypothetical model that emits text then a think block mid-stream would render the second block as literal HTML. This is consistent with how these models actually work (reasoning is always emitted first) and renderMd() handles unrecognized tags safely.

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Browser feature test: PASS

Tested on the PR branch (fix/thinking-display-edge-cases) with a live server on port 8789:

1. Stored session rendering — injected a session with two assistant messages:

   • <think>The user asks a simple arithmetic question...</think>\n\nThe answer is **4**.
   • <|channel>thought\nThe user asks about European capitals...\n<channel|>\n\nThe capital of France is **Paris**.

   Both rendered as collapsible gold "💡 Thinking ▸" cards. The think/channel content was stripped from the visible response body. Expanding the first card showed the verbatim reasoning text. ✅

2. Live streaming — sent a message via the UI against the mock provider (port 8793, no real API key). Full SSE pipeline ran end-to-end; response streamed and rendered correctly. ✅

No console errors observed.

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No fixes required. The PR is clean, correct, and well-scoped. Safe to merge.