fix(#4346): suppress footer jitter during virtual scroll measurement re-renders

tests/test_issue4346_vscroll_footer_jitter.py

@@ -0,0 +1,50 @@
+"""Static source assertions for issue #4346 virtual-scroll footer jitter fix."""
+import pathlib
+import re
+
+ROOT = pathlib.Path(__file__).parent.parent
+
+CSS = (ROOT / 'static' / 'style.css').read_text(encoding='utf-8')
+JS  = (ROOT / 'static' / 'ui.js').read_text(encoding='utf-8')
+
+
+def test_css_vscroll_measuring_guard():
+    """style.css suppresses opacity transitions on .msg-foot and .msg-actions
+    while .vscroll-measuring is present on the scroll container."""
+    assert 'vscroll-measuring' in CSS
+    assert re.search(
+        r'\.vscroll-measuring\s+\.msg-foot.*transition\s*:\s*none\s*!important',
+        CSS, re.DOTALL
+    ), "missing transition:none !important for .vscroll-measuring .msg-foot"
+    assert re.search(
+        r'\.vscroll-measuring\s+\.msg-actions.*transition\s*:\s*none\s*!important',
+        CSS, re.DOTALL
+    ), "missing transition:none !important for .vscroll-measuring .msg-actions"
+
+
+def test_js_compensate_adds_vscroll_measuring():
+    """_compensateScrollForMeasurementDelta adds and removes the vscroll-measuring
+    class around the render callback."""
+    fn_match = re.search(
+        r'function _compensateScrollForMeasurementDelta\(renderFn\)\{(.+?)^(?=function )',
+        JS, re.DOTALL | re.MULTILINE
+    )
+    assert fn_match, "_compensateScrollForMeasurementDelta not found"
+    body = fn_match.group(1)
+    assert "classList.add('vscroll-measuring')" in body
+    assert "classList.remove('vscroll-measuring')" in body
+
+
+def test_js_try_finally_guards_class_removal():
+    """The classList.remove is in a finally block so the class is always cleared."""
+    fn_match = re.search(
+        r'function _compensateScrollForMeasurementDelta\(renderFn\)\{(.+?)^(?=function )',
+        JS, re.DOTALL | re.MULTILINE
+    )
+    assert fn_match
+    body = fn_match.group(1)
+    add_pos = body.find("classList.add('vscroll-measuring')")
+    try_pos = body.find('try{', add_pos) if add_pos != -1 else -1
+    finally_pos = body.find('finally{', try_pos) if try_pos != -1 else -1
+    remove_pos = body.find("classList.remove('vscroll-measuring')", finally_pos) if finally_pos != -1 else -1
+    assert remove_pos != -1, "classList.remove must be inside a finally block after classList.add"