Fix PWA final answer scroll refresh

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Long final answers no longer snap back to the bottom while you are reading them in PWA/mobile mode (#4123).** Same-session background refreshes now preserve a reader's current transcript position whenever the viewport is clearly away from the bottom, and they no longer clear the sticky scroll-unpin state before rebuilding the transcript. (#4123)
+
 ## [v0.51.388] — 2026-06-13 — Release NA (Stable Assistant Turn Anchors dual-run reconciler, inert, #3926)
 
 ### Added

static/sessions.js

@@ -1038,10 +1038,13 @@ async function loadSession(sid){
   if(typeof _hydrateTodosFromSession==='function') _hydrateTodosFromSession(S.session);
   S.session._modelResolutionDeferred=true;
   S.lastUsage={...(data.session.last_usage||{})};
-  // Reset scroll-direction tracker on session switch so the new chat's
-  // first scroll doesn't compare against the previous chat's scrollTop
+  // Reset scroll-direction tracker only on real session switches so the new
+  // chat's first scroll doesn't compare against the previous chat's scrollTop
   // and false-trigger an unpin (#1731 follow-up — Opus stage-302 SHOULD-FIX).
-  if (typeof window !== 'undefined' && typeof window._resetScrollDirectionTracker === 'function') {
+  // Same-session force refreshes reuse the current transcript viewport; clearing
+  // the sticky-unpin state here makes preserveScroll treat a reader mid-answer
+  // as pinned and snap them back to the bottom on the next render.
+  if (currentSid !== sid && typeof window !== 'undefined' && typeof window._resetScrollDirectionTracker === 'function') {
     try { window._resetScrollDirectionTracker(); } catch (_) {}
   }
   if(typeof _applyPendingSessionModelForSession==='function') _applyPendingSessionModelForSession(sid);

static/ui.js

@@ -8718,6 +8718,16 @@ function _restoreMessageScrollSnapshot(snapshot){
   el.scrollTop=Math.max(0,Math.min(Number(snapshot.top)||0,maxTop));
   // Sync _lastScrollTop after programmatic restore so sticky-unpin does not false-trigger (#1731).
   _lastScrollTop=el.scrollTop;
+  const bottomDistance=el.scrollHeight-el.scrollTop-el.clientHeight;
+  if(bottomDistance>250){
+    _messageUserUnpinned=true;
+    _scrollPinned=false;
+    _nearBottomCount=0;
+  }else if(bottomDistance<=120){
+    _messageUserUnpinned=false;
+    _scrollPinned=true;
+    _nearBottomCount=2;
+  }
   requestAnimationFrame(()=>{ setTimeout(()=>{_programmaticScroll=false;},0); });
 }
 function _restoreMessageScrollSnapshotSameFrame(snapshot){
@@ -8773,6 +8783,11 @@ function _scrollAfterMessageRender(preserveScroll, scrollSnapshot){
   // pinned users stay at bottom; users who manually scrolled up get their
   // pre-render scrollTop restored after the DOM replacement.
   if(preserveScroll){
+    const readerAwayFromBottom=!!(
+      scrollSnapshot &&
+      Number.isFinite(Number(scrollSnapshot.bottom)) &&
+      Number(scrollSnapshot.bottom)>250
+    );
     // Keep master's follow heuristic for pinned / still-near-bottom users:
     // _followMessagesAfterDomReplace() does a FORCED scrollToBottom() (synchronous
     // bottom write + forced settle), so the final settled response can't leave a
@@ -8781,7 +8796,7 @@ function _scrollAfterMessageRender(preserveScroll, scrollSnapshot){
     // new-message cue. (Using scrollIfPinned() here instead would skip the forced
     // write unless distance>500 and let the DOM-rebuild scroll event cancel the
     // delayed settles — Codex CORE catch on #3631.)
-    if(!_messageUserUnpinned && _followMessagesAfterDomReplace()) return;
+    if(!readerAwayFromBottom && !_messageUserUnpinned && _followMessagesAfterDomReplace()) return;
     _restoreMessageScrollSnapshot(scrollSnapshot);
     _maybeShowNewMessageScrollCue(scrollSnapshot);
     return;

tests/test_issue3479_ios_stream_scroll_jump.py

@@ -10,6 +10,7 @@ from pathlib import Path
 
 ROOT = Path(__file__).resolve().parents[1]
 UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+SESSIONS_JS = (ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
 STYLE_CSS = (ROOT / "static" / "style.css").read_text(encoding="utf-8")
 
 
@@ -78,6 +79,36 @@ def test_same_frame_snapshot_preserves_bottom_distance_and_unpinned_state():
     assert "_restoreMessageScrollSnapshotSameFrame(scrollSnapshot);" in wrapper
 
 
+def test_preserve_scroll_restores_reader_away_from_bottom_before_following():
+    body = _function_body(UI_JS, "_scrollAfterMessageRender")
+    compact = _compact(body)
+
+    reader_idx = compact.index("constreaderAwayFromBottom=")
+    follow_idx = compact.index("if(!readerAwayFromBottom&&!_messageUserUnpinned&&_followMessagesAfterDomReplace())return;")
+    restore_idx = compact.index("_restoreMessageScrollSnapshot(scrollSnapshot);")
+
+    assert "Number(scrollSnapshot.bottom)>250" in compact
+    assert reader_idx < follow_idx < restore_idx
+
+
+def test_scroll_snapshot_restore_reinstates_unpinned_state_when_reader_is_mid_answer():
+    restore = _function_body(UI_JS, "_restoreMessageScrollSnapshot")
+    compact = _compact(restore)
+
+    assert "constbottomDistance=el.scrollHeight-el.scrollTop-el.clientHeight;" in compact
+    assert "if(bottomDistance>250)" in compact
+    assert "_messageUserUnpinned=true" in compact
+    assert "_scrollPinned=false" in compact
+
+
+def test_same_session_force_refresh_does_not_reset_scroll_direction_tracker():
+    body = _function_body(SESSIONS_JS, "loadSession")
+    compact = _compact(body)
+
+    assert "constcurrentSid=S.session?S.session.session_id:null;" in compact
+    assert "if(currentSid!==sid&&typeofwindow!=='undefined'&&typeofwindow._resetScrollDirectionTracker==='function')" in compact
+
+
 def test_clarify_card_is_height_clamped_and_scrollable_on_mobile_viewports():
     compact = _compact(STYLE_CSS)