fix(streaming): restore mobile scroll-jank guard during live renders

A later streaming parse-cache change dropped the #MOBILESCROLL guard from
the live-stream render tick while leaving the helper and CSS in place.
On iOS PWA, Safari can paint a transient scrollTop=0 frame during streamed
DOM rebuilds, forcing the reader to the first/oldest message.

Re-add _fixMobileScrollJank() before streaming DOM work begins and pin it
with a regression test so the third layer of the mobile scroll fix cannot
be silently dropped again.

static/messages.js

@@ -3844,6 +3844,9 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
       _renderPending=false;
       // Guard: a pending setTimeout+rAF can outlive stream finalization.
       if(_streamFinalized) return;
+      // Mobile scroll-jank guard: enable overflow-anchor before DOM writes so
+      // the browser preserves scroll position during streaming content growth.
+      if(typeof window._fixMobileScrollJank==='function') window._fixMobileScrollJank();
       _lastRenderMs=performance.now();
       const parsed=_cachedParsed&&_cachedParsedText===assistantText&&_cachedParsedReasoning===liveReasoningText ? _cachedParsed : _parseStreamState();
       _cachedParsed=null;

tests/test_issue1360_streaming_scroll_hardening.py

@@ -4,6 +4,7 @@ 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")
 STYLE_CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
 
 
@@ -43,6 +44,28 @@ def test_messages_scroller_uses_overflow_anchor_auto_on_mobile():
     )
 
 
+def test_streaming_render_enables_mobile_scroll_jank_guard_before_dom_writes():
+    """Streaming must re-enable mobile scroll anchoring before every DOM write.
+
+    Regression: #MOBILESCROLL originally added _fixMobileScrollJank() in the
+    live-stream render tick, but a later streaming parse-cache change dropped
+    that call while leaving the helper/CSS in place. On iOS PWA this lets Safari
+    paint a transient scrollTop=0 frame during streamed DOM rebuilds.
+    """
+    guard_idx = MESSAGES_JS.find("window._fixMobileScrollJank")
+    assert guard_idx != -1, (
+        "attachLiveStream() must call window._fixMobileScrollJank() during the "
+        "streaming render tick, before DOM writes, so iOS PWA does not jump to "
+        "the first/oldest message while assistant output streams."
+    )
+
+    render_idx = MESSAGES_JS.find("_lastRenderMs=performance.now()")
+    assert render_idx != -1, "streaming render timestamp not found"
+    assert guard_idx < render_idx, (
+        "The mobile scroll-jank guard must run before streaming DOM work begins."
+    )
+
+
 def test_scroll_repin_dead_zone_is_wider_for_mac_app_windows():
     assert "clientHeight<250" in UI_JS or "bottomDistance<250" in UI_JS, (
         "The near-bottom re-pin threshold should be at least 250px so small "