perf(#4346): recycle stable DOM nodes during virtual-scroll measurement re-renders

# Conflicts:
#	static/ui.js
#	tests/test_issue4346_vscroll_footer_jitter.py

tests/test_issue4346_vscroll_footer_jitter.py

@@ -51,3 +51,55 @@ def test_js_try_finally_guards_class_removal():
     assert remove_idx != -1, "missing classList.remove('vscroll-measuring')"
     assert try_idx < finally_idx < remove_idx, \
         "classList.remove must remain in the finally{} cleanup path"
+
+
+def test_js_recycle_flag_exists():
+    """ui.js declares the _msgNodeRecycleEnabled flag."""
+    assert '_msgNodeRecycleEnabled' in JS
+
+
+def test_js_recycle_stash_exists():
+    """ui.js declares the _recycleStash Map."""
+    assert '_recycleStash' in JS
+
+
+def test_js_recycle_flag_set_in_virtual_render():
+    """_scheduleMessageVirtualizedRender sets _msgNodeRecycleEnabled=true
+    before the compensate call and clears it in finally."""
+    fn_match = re.search(
+        r'function _scheduleMessageVirtualizedRender\(force\)\{(.+?)^(?=function )',
+        JS, re.DOTALL | re.MULTILINE
+    )
+    assert fn_match, "_scheduleMessageVirtualizedRender not found"
+    body = fn_match.group(1)
+    assert '_msgNodeRecycleEnabled=true' in body
+    finally_match = re.search(r'finally\{([^}]*)\}', body)
+    assert finally_match, "no finally block in _scheduleMessageVirtualizedRender"
+    assert '_msgNodeRecycleEnabled=false' in finally_match.group(1)
+
+
+def test_js_stash_populated_before_wipe():
+    """The recycleStash population loop appears before innerHTML='' in renderMessages."""
+    fn_match = re.search(
+        r'function renderMessages\(options\)\{(.+?)^(?=function )',
+        JS, re.DOTALL | re.MULTILINE
+    )
+    assert fn_match, "renderMessages not found"
+    body = fn_match.group(1)
+    stash_pos = body.find('_recycleStash.set(')
+    wipe_pos = body.find("inner.innerHTML='';")
+    assert stash_pos != -1, "_recycleStash.set not found in renderMessages"
+    assert wipe_pos != -1, "innerHTML wipe not found in renderMessages"
+    assert stash_pos < wipe_pos, "_recycleStash.set must appear before innerHTML=''"
+
+
+def test_js_user_row_checks_stash():
+    """The user-row creation block checks _recycleStash before createElement."""
+    fn_match = re.search(
+        r'function renderMessages\(options\)\{(.+?)^(?=function )',
+        JS, re.DOTALL | re.MULTILINE
+    )
+    assert fn_match, "renderMessages not found"
+    body = fn_match.group(1)
+    assert '_recycleStash.get(rawIdx)' in body, \
+        "user row block must check _recycleStash.get(rawIdx)"