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_issue734_message_windowing.py

@@ -1,26 +1,30 @@
 from pathlib import Path
 
 REPO = Path(__file__).resolve().parents[1]
-UI_JS = (REPO / "static" / "ui.js").read_text()
-CSS = (REPO / "static" / "style.css").read_text()
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+SESSIONS_JS = (REPO / "static" / "sessions.js").read_text(encoding="utf-8")
+CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
 
 
-def test_message_windowing_caps_initial_dom_to_recent_messages():
-    assert "const MESSAGE_RENDER_WINDOW_DEFAULT=50" in UI_JS
-    assert "_messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT" in UI_JS
-    assert "const windowStart=Math.max(0, visWithIdx.length-renderWindowSize)" in UI_JS
-    assert "const renderVisWithIdx=visWithIdx.slice(windowStart)" in UI_JS
-    assert "for(let vi=0;vi<renderVisWithIdx.length;vi++)" in UI_JS
-    assert "Load earlier messages (${hiddenBeforeCount} hidden)" in UI_JS
+def test_message_virtualization_switches_render_messages_to_scroll_driven_window():
+    assert "function _messageVirtualWindow(opts)" in UI_JS
+    assert "function _messageVirtualKeepTailCount()" in UI_JS
+    assert "const virtualWindow=_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount())" in UI_JS
+    assert "const renderHeadVisWithIdx=visWithIdx.slice(windowStart, windowEnd)" in UI_JS
+    assert "const renderTailStart=virtualWindow.virtualized?Math.max(windowEnd, virtualWindow.tailStart):windowEnd" in UI_JS
+    assert "const renderTailVisWithIdx=virtualWindow.virtualized&&renderTailStart<visWithIdx.length" in UI_JS
+    assert "const renderVisWithIdx=renderHeadVisWithIdx.concat(renderTailVisWithIdx)" in UI_JS
+    assert "if(virtualWindow.virtualized&&virtualWindow.bottomPad>0&&vi===headRenderCount)" in UI_JS
 
 
-def test_load_earlier_expands_local_window_before_server_pagination_and_preserves_scroll():
-    assert "function _showEarlierRenderedMessages()" in UI_JS
-    assert "prevScrollH=container?container.scrollHeight:0" in UI_JS
-    assert "prevScrollTop=container?container.scrollTop:0" in UI_JS
-    assert "container.scrollTop=prevScrollTop+(newScrollH-prevScrollH)" in UI_JS
-    assert "if(_messageHiddenBeforeCount()>0) _showEarlierRenderedMessages();" in UI_JS
-    assert "else if(typeof _loadOlderMessages==='function') _loadOlderMessages();" in UI_JS
+def test_load_earlier_only_pages_server_history_and_preserves_scroll():
+    assert "function _wireMessageWindowLoadEarlierButton()" in UI_JS
+    assert "if(typeof _loadOlderMessages==='function') _loadOlderMessages();" in UI_JS
+    assert "if(hasServerOlder){" in UI_JS
+    assert "if(virtualWindow.virtualized&&virtualWindow.topPad>0)" in UI_JS
+    assert "_messageRenderWindowSize=_currentMessageRenderWindowSize()+Math.max(addedRenderable, MESSAGE_RENDER_WINDOW_DEFAULT);" in SESSIONS_JS
+    assert "renderMessages({ preserveScroll: true });" in SESSIONS_JS
+    assert "_scheduleMessageVirtualizedRender();" in UI_JS
 
 
 def test_windowed_render_keeps_streaming_and_tool_activity_anchored_to_rendered_messages():
@@ -32,15 +36,26 @@ def test_windowed_render_keeps_streaming_and_tool_activity_anchored_to_rendered_
 
 
 def test_window_state_participates_in_cache_and_cached_button_is_rewired():
-    assert "cached.renderWindowSize===renderWindowSize" in UI_JS
+    assert "cached.renderWindowKey===renderWindowKey" in UI_JS
     assert "cached.signature===renderSignature" in UI_JS
-    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize,signature:renderSignature})" in UI_JS
+    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowKey,signature:renderSignature})" in UI_JS
+    assert "_messageVirtualWindowKey=renderWindowKey" in UI_JS
     assert "function _wireMessageWindowLoadEarlierButton()" in UI_JS
     assert "_wireMessageWindowLoadEarlierButton();" in UI_JS
     assert UI_JS.count("_wireMessageWindowLoadEarlierButton();") >= 2
 
 
-def test_load_earlier_affordance_has_button_styling_hook():
+def test_virtualization_affordances_have_styling_hooks():
     assert "message-window-load-earlier" in UI_JS
     assert ".message-window-load-earlier" in CSS
+    assert ".message-virtual-spacer" in CSS
     assert "border-radius:999px" in CSS
+
+
+def test_measurement_rerenders_are_bounded_per_virtual_window_cycle():
+    assert "const MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS=2;" in UI_JS
+    assert "function _messageVirtualMeasurementCycleKeyFor(windowMetrics)" in UI_JS
+    assert "function _scheduleMessageVirtualMeasurementRefresh(windowMetrics)" in UI_JS
+    assert "if(_messageVirtualMeasurementRetryCount>=MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS) return;" in UI_JS
+    assert "_scheduleMessageVirtualMeasurementRefresh(virtualWindow);" in UI_JS
+    assert "_markMessageVirtualMeasurementsSettled(virtualWindow);" in UI_JS