fix(#4856): suppress Chromium scroll-anchor re-selection during mobile DOM rebuild

static/ui.js

@@ -11029,22 +11029,19 @@ function _restoreMessageScrollSnapshot(snapshot){
   }
 }
 /**
- * Mobile scroll-jank guard: temporarily re-enable overflow-anchor so the
- * browser preserves scroll position across the innerHTML='' + DOM rebuild gap.
- * Safari/Chrome on iOS/Android can paint a frame with scrollTop=0 between
- * innerHTML='' and snapshot restore, scroll-janking the user to the top.
- * Called from renderMessages() before the DOM wipe — not from streaming ticks,
- * since CSS already gives overflow-anchor:auto on mobile via media query.
+ * Mobile scroll-jank guard: temporarily disable overflow-anchor so
+ * Chromium cannot re-anchor to the topmost row during the innerHTML=''
+ * wipe-and-rebuild gap. The rAF callback restores CSS default afterward.
  */
 window._fixMobileScrollJank=function _fixMobileScrollJank(){
   const el=document.getElementById('messages');
   if(!el) return;
   // Desktop with a mouse: keep overflow-anchor:none (explicitly set by CSS).
-  // Mobile touch devices only: temporarily enable it.
+  // Mobile touch devices only: temporarily suppress anchor re-selection.
   if(window.matchMedia('(hover:hover) and (pointer:fine)').matches) return;
-  el.style.overflowAnchor='auto';
+  el.style.overflowAnchor='none';
   requestAnimationFrame(()=>{
-    if(el.style.overflowAnchor==='auto') el.style.overflowAnchor='';
+    if(el.style.overflowAnchor==='none') el.style.overflowAnchor='';
   });
 };
 
@@ -11282,10 +11279,8 @@ function renderMessages(options){
       _recycleStash.set(Number(key), child);
     }
   }
-  // Mobile scroll-jank fix: temporarily re-enable overflow-anchor so browser
-  // preserves scroll position across the DOM wipe-and-rebuild gap.
-  // Safari/Chrome on iOS/Android can paint a frame with scrollTop=0 between
-  // innerHTML='' and snapshot restore, scroll-janking the user to the top.
+  // Mobile scroll-jank fix: temporarily disable overflow-anchor so Chromium
+  // cannot re-anchor to the topmost row during the DOM wipe-and-rebuild gap.
   if(window._fixMobileScrollJank) window._fixMobileScrollJank();
   inner.innerHTML='';
   const compressionNode=compressionState?_compressionCardsNode(compressionState):null;

tests/test_issue4856_android_scroll_regression.py

@@ -0,0 +1,98 @@
+"""Regression tests for #4856: Android scroll-to-top on every interaction."""
+
+from pathlib import Path
+
+REPO = Path(__file__).parent.parent
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+MESSAGES_JS = (REPO / "static" / "messages.js").read_text(encoding="utf-8")
+
+_FUNC_MARKER = "window._fixMobileScrollJank=function _fixMobileScrollJank(){"
+_RAF_MARKER = "requestAnimationFrame(()=>{"
+
+
+def _extract_fix_mobile_scroll_jank(src: str) -> str:
+    idx = src.find(_FUNC_MARKER)
+    assert idx != -1, "_fixMobileScrollJank not found in ui.js"
+    depth = 0
+    for i, ch in enumerate(src[idx:], idx):
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[idx : i + 1]
+    raise AssertionError("Could not extract _fixMobileScrollJank")
+
+
+def _extract_raf_body(fn_src: str) -> str:
+    idx = fn_src.find(_RAF_MARKER)
+    assert idx != -1, "requestAnimationFrame callback not found in function"
+    depth = 0
+    for i, ch in enumerate(fn_src[idx:], idx):
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return fn_src[idx : i + 1]
+    raise AssertionError("Could not extract rAF body")
+
+
+def test_fix_sets_none_not_auto():
+    # Base-fails/head-passes: function must set 'none' to suppress Chromium
+    # scroll-anchor re-selection; 'auto' is already the CSS default so setting
+    # it is a no-op and leaves the anchor engine active during the DOM wipe.
+    fn = _extract_fix_mobile_scroll_jank(UI_JS)
+    assert "overflowAnchor='none'" in fn, (
+        "_fixMobileScrollJank() must set overflowAnchor='none' to suppress "
+        "Chromium scroll-anchor re-selection during the DOM wipe (#4856)."
+    )
+    assert "overflowAnchor='auto'" not in fn, (
+        "_fixMobileScrollJank() must not set overflowAnchor='auto'; that is "
+        "already the CSS resting value on mobile and is a no-op."
+    )
+
+
+def test_raf_cleanup_checks_none():
+    # The rAF guard must check for 'none' so it only clears the inline style
+    # it set; checking 'auto' was always false and left the inline style on.
+    fn = _extract_fix_mobile_scroll_jank(UI_JS)
+    raf = _extract_raf_body(fn)
+    assert "overflowAnchor==='none'" in raf, (
+        "The rAF cleanup in _fixMobileScrollJank() must check for 'none' so it "
+        "clears the inline style after the synchronous scrollTop write lands."
+    )
+    assert "overflowAnchor==='auto'" not in raf, (
+        "The rAF cleanup must not check for 'auto'; that check was always false."
+    )
+
+
+def test_rebuild_path_calls_fix_before_wipe():
+    # renderMessages() must call _fixMobileScrollJank() before innerHTML='' so
+    # anchor suppression is active during the full wipe-and-rebuild window.
+    fix_idx = UI_JS.find("window._fixMobileScrollJank()")
+    assert fix_idx != -1, (
+        "renderMessages() must call window._fixMobileScrollJank() before innerHTML=''."
+    )
+    wipe_idx = UI_JS.find("innerHTML=''", fix_idx)
+    assert wipe_idx != -1, (
+        "innerHTML='' not found after _fixMobileScrollJank() call site."
+    )
+    assert fix_idx < wipe_idx, (
+        "_fixMobileScrollJank() must be called before innerHTML='' in renderMessages()."
+    )
+
+
+def test_streaming_tick_calls_fix_before_dom_writes():
+    # The streaming render tick in messages.js must call _fixMobileScrollJank()
+    # before _lastRenderMs=performance.now() so anchor suppression covers every
+    # incremental DOM update during streaming.
+    guard_idx = MESSAGES_JS.find("window._fixMobileScrollJank")
+    assert guard_idx != -1, (
+        "The streaming tick must call window._fixMobileScrollJank() before DOM writes."
+    )
+    render_idx = MESSAGES_JS.find("_lastRenderMs=performance.now()")
+    assert render_idx != -1, "streaming render timestamp not found in messages.js"
+    assert guard_idx < render_idx, (
+        "The mobile scroll-jank guard must run before streaming DOM work begins."
+    )