fix(#5544): re-pin auto-scroll only at true bottom after scroll-to-bottom

A single scroll-up during streaming set _messageUserUnpinned=true and
scrollIfPinned() then permanently stopped auto-follow (only scrollToBottom()
cleared it) — a permanent auto-follow lockout even after the user returned to
the bottom. scrollIfPinned() now re-pins, but ONLY when the reader has genuinely
reached the true-bottom tail (<=80px) AND shows no active scroll intent
(wheel/key/touch/non-message), reusing the listener's _nearBottomCount debounce.
Proximity alone (the ~250px nearBottom band) must never re-pin — that is the
#4295 invariant. Restores the listener's <=80px true-bottom gate too.

Co-authored-by: luperrypf <luperrypf@users.noreply.github.com>

static/ui.js

@@ -4858,6 +4858,10 @@ if(typeof window!=='undefined'){
       }else if(movedDown&&nearBottom){
         _nearBottomCount=_nearBottomCount+1;
         if(_nearBottomCount>=2){
+          // Only re-pin when the reader has genuinely reached the true bottom
+          // tail (<=80px). nearBottom spans a ~250px band, so proximity alone
+          // must NOT clear the sticky unpin flag (#4295) — a reader scanning the
+          // last lines mid-stream would otherwise get yanked back to the bottom.
           if(!_messageUserUnpinned||bottomDistance<=80){
             _messageUserUnpinned=false;
             _scrollPinned=true;
@@ -5516,7 +5520,24 @@ function _settleFinalScroll(token){
 }
 function scrollIfPinned(){
   if(!_autoScrollFollow) return;
-  if(_messageUserUnpinned) return;
+  if(_messageUserUnpinned){
+    // Only scrollToBottom() cleared this flag, so one scroll-up permanently
+    // killed auto-follow. Re-pin ONLY when the reader has genuinely returned to
+    // the true bottom tail (<=80px), NOT on mere near-bottom proximity — the
+    // #4295 invariant is that proximity alone (inside the ~250px band) must not
+    // re-pin, or a reader scanning the last few lines gets yanked to the bottom
+    // mid-stream. Also bail on ANY recent message-pane scroll intent (wheel,
+    // key, touch) and non-message intent, so an active scroll-up near the tail
+    // is never overridden. Uses the same _nearBottomCount debounce as the
+    // scroll listener (~4859-4866).
+    if(_recentNonMessageScrollIntent()||_recentMessageScrollIntent()||_recentMessageTouchScrollIntent()||_recentMessageWheelIntent()||_recentMessageKeyScrollIntent()){ _nearBottomCount=0; return; }
+    if(_messageBottomDistance()>80){ _nearBottomCount=0; return; }
+    _nearBottomCount=_nearBottomCount+1;
+    if(_nearBottomCount<2) return;
+    _nearBottomCount=0;
+    _messageUserUnpinned=false;
+    _scrollPinned=true;
+  }
   if(!_scrollPinned) return;
   if(_recentNonMessageScrollIntent()) return;
   if(_messageBottomDistance()>500) _setMessageScrollToBottom();

tests/test_issue3250_upward_scroll_intent_window.py

@@ -32,8 +32,19 @@ def _scroll_listener_block() -> str:
 def test_scroll_if_pinned_respects_sticky_user_unpin():
     fn = UI_JS[UI_JS.index("function scrollIfPinned"): UI_JS.index("function scrollToBottom")]
     compact = fn.replace(" ", "")
-    assert "if(_messageUserUnpinned)return" in compact, (
-        "scrollIfPinned() must not fight a sticky manual unpin during streaming"
+    # scrollIfPinned() re-pins auto-follow when the reader returns to the tail
+    # (#5544), but it must still RESPECT a sticky manual unpin: the re-pin is
+    # gated behind the true-bottom threshold (<=80px, not mere proximity) and
+    # bails on any recent scroll intent, so it never fights a reader who has
+    # scrolled up. Assert the guards rather than the old unconditional bail.
+    assert "if(_messageUserUnpinned){" in compact, (
+        "scrollIfPinned() must handle the sticky-unpin case explicitly"
+    )
+    assert "_messageBottomDistance()>80" in compact, (
+        "re-pin must require the true bottom tail (<=80px), not proximity — the #4295 invariant"
+    )
+    assert "_recentMessageWheelIntent()" in compact and "_recentMessageKeyScrollIntent()" in compact, (
+        "re-pin must bail on recent wheel/key scroll intent so it can't fight a manual scroll-up"
     )
 
 

tests/test_tars_scroll_reset_regressions.py

@@ -100,7 +100,18 @@ def test_user_scroll_cancels_delayed_bottom_settling():
     assert "_cancelBottomSettle();" in record
     assert "_lastNonMessageScrollIntentMs=performance.now();" in record
     assert "_scrollPinned=false" in record
-    assert "if(_messageUserUnpinned) return;" in pinned
+    # scrollIfPinned() may now re-pin auto-follow (#5544), but ONLY when the
+    # reader has genuinely returned to the true bottom tail (<=80px) AND no
+    # recent scroll intent is active — it must never fight a manual scroll-up.
+    # Assert those guards are present instead of the old "bail unconditionally
+    # when unpinned" lock.
+    _pinned_compact = pinned.replace(" ", "")
+    assert "_messageBottomDistance()>80" in _pinned_compact, (
+        "scrollIfPinned() re-pin must require the true bottom tail (<=80px), not mere proximity"
+    )
+    assert "_recentMessageWheelIntent()" in _pinned_compact and "_recentMessageKeyScrollIntent()" in _pinned_compact, (
+        "scrollIfPinned() re-pin must bail on recent wheel/key scroll intent so it can't fight a reader"
+    )
     assert "_messageUserUnpinned" in final and "return" in final
     assert "_recentMessageUpwardIntent()" not in pinned