perf(chat): virtualize long message transcripts (#500) (#4214)

* perf(chat): virtualize long message transcripts (#500)

* fix(chat): preserve virtual height cache on append (#500)

* fix(chat): bound virtual tail follow-up (#500)

* fix(chat): surface older history and bound virtual rerenders (#500)

* fix(chat): stabilize virtual history fallback (#500)

* fix(chat): preserve virtual compression anchors (#500)

* Keep virtualization regressions aligned with rebased sources (#500)

* fix(chat): preserve virtual heights across prepends (#500)

* test(update): isolate restart timing checks (#814)

* fix(chat): restore render contracts around virtual cache

* fix(chat): preserve legacy assistant anchor predicates

* fix(#3714): restore worklog-disclosure capture after HTML-cache early-return

Deep-review finding: the capture of _captureWorklogDetailDisclosureState was
moved before the HTML-cache fast-path return, failing the invariant test
test_render_rebuild_preserves_worklog_detail_disclosure_click_state (it must
not traverse the previous-session DOM when the cache path returns early).
Moved it back to just before inner.innerHTML='' (after the cache return).

Co-authored-by: Rod Boev <rod.boev@gmail.com>

* fix(#3714): restore Transparent Stream rehydration in HTML-cache fast path

Full-suite gate caught test_issue3820_chat_activity_display_mode failure: the
virtualization rewrite of the renderMessages HTML-cache fast-path block dropped
the _rehydrateTransparentStreamDom(inner) call (master has it; PR had 2 of 3
occurrences). On a cache-restore session switch, Transparent Stream controls
lost their property-assigned handlers. Restored the call in the same position
master has it (right after _sessionHtmlCacheSid=sid).

Co-authored-by: Rod Boev <rod.boev@gmail.com>

* fix(#3714): preserve live turn on scroll-rebuild even with zero streamed text

Opus ship-review residual (now fixed, not deferred): the live-turn re-attach
after a scroll-triggered virtualized rebuild was gated on _preservedLen>0, so a
turn showing only Activity/tool/worklog blocks with no streamed text yet (mid
tool-call) would not be preserved — scrolling up into history on a >80-row
transcript mid-stream could blink those live-only blocks for a frame. Broaden
the gate to also restore when the preserved turn has MORE structural blocks than
the rebuilt (lagging-S.messages) turn. Hoisted the _structuralCount helper above
the gate; whole-turn-vs-segment restore logic unchanged.

Co-authored-by: Rod Boev <rod.boev@gmail.com>

* chore(release): stamp v0.51.423 (Release OJ) for #500 transcript virtualization

---------

Co-authored-by: Rod Boev <rod.boev@gmail.com>
Co-authored-by: nesquena-hermes <agent@nesquena-hermes>

tests/test_issue401.py

@@ -72,14 +72,16 @@ def test_loadsession_uses_session_toolcalls_only_as_fallback():
 
 def test_rendermessages_treats_openai_toolcall_assistants_as_visible():
     """OpenAI assistant rows with empty content but tool_calls must stay anchorable."""
+    assert "function _messageIsRenderable(m)" in UI_JS
     assert "const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;" in UI_JS
-    assert "if(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)) return true;" in UI_JS
+    assert "hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)" in UI_JS
 
 
 def test_rendermessages_treats_partial_toolcall_assistants_as_visible():
     """Assistant rows carrying `_partial_tool_calls` must stay anchorable."""
+    assert "function _messageIsRenderable(m)" in UI_JS
     assert "const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;" in UI_JS
-    assert "if(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)) return true;" in UI_JS
+    assert "hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)" in UI_JS
 
 
 def test_rendermessages_rebuilds_tool_cards_from_partial_tool_calls():