Harden streaming scroll unpin behavior (#1360) (#1377)

static/style.css

@@ -681,7 +681,7 @@
   .workspace-toggle-btn.active{color:var(--accent-text);border-color:var(--accent-bg);background:var(--accent-bg);}
   .workspace-toggle-btn:disabled{opacity:.38;cursor:not-allowed;}
   .chip.model{color:var(--accent-text);border-color:var(--accent-bg-strong);background:var(--accent-bg);}
-  .messages{flex:1;overflow-y:auto;display:flex;flex-direction:column;min-height:0;position:relative;z-index:0;-webkit-overflow-scrolling:touch;touch-action:pan-y;overscroll-behavior-y:contain;}
+  .messages{flex:1;overflow-y:auto;display:flex;flex-direction:column;min-height:0;position:relative;z-index:0;-webkit-overflow-scrolling:touch;touch-action:pan-y;overscroll-behavior-y:contain;overflow-anchor:none;}
   /* sticky-first-child: button is first child of .messages so its natural position is above viewport; sticky+bottom:16px pins it there when visible */
   .scroll-to-bottom-btn{position:sticky;bottom:16px;align-self:flex-end;margin-right:20px;width:32px;height:32px;border-radius:50%;border:1px solid var(--border2);background:var(--code-bg);color:var(--muted);font-size:16px;cursor:pointer;display:flex;align-items:center;justify-content:center;box-shadow:0 2px 8px rgba(0,0,0,.25);z-index:10;transition:color .12s,border-color .12s,background .12s;}
   .scroll-to-bottom-btn:hover{color:var(--text);border-color:var(--border);background:var(--hover-bg);}

static/ui.js

@@ -827,13 +827,13 @@ document.addEventListener('click',function(e){
 
 // ── Scroll pinning ──────────────────────────────────────────────────────────
 // When streaming, auto-scroll only if the user hasn't manually scrolled up.
-// Once the user scrolls back to within 150px of the bottom, re-pin.
+// Once the user scrolls back to within 250px of the bottom, re-pin.
 let _scrollPinned=true;
 (function(){
   const el=document.getElementById('messages');
   if(!el) return;
   el.addEventListener('scroll',()=>{
-    const nearBottom=el.scrollHeight-el.scrollTop-el.clientHeight<150;
+    const nearBottom=el.scrollHeight-el.scrollTop-el.clientHeight<250;
     _scrollPinned=nearBottom;
     const btn=$('scrollToBottomBtn');
     if(btn) btn.style.display=_scrollPinned?'none':'flex';
@@ -1757,7 +1757,8 @@ function _renderQueueChips(sid){
       if(!card.classList.contains('visible')) return;
       const h=card.getBoundingClientRect().height;
       if(h>0) _msgs.style.setProperty('--queue-card-height', h+'px');
-      if(typeof scrollToBottom==='function') scrollToBottom();
+      if(S.activeStreamId&&typeof scrollIfPinned==='function') scrollIfPinned();
+      else if(!S.activeStreamId&&typeof scrollToBottom==='function') scrollToBottom();
     }, 360);
   }
 
@@ -1946,7 +1947,8 @@ function _updateQueuePill(sid,count){
         }, 360);
       }
       if(pillOuter) pillOuter.classList.remove('show');
-      if(typeof scrollToBottom==='function') scrollToBottom();
+      if(S.activeStreamId&&typeof scrollIfPinned==='function') scrollIfPinned();
+      else if(!S.activeStreamId&&typeof scrollToBottom==='function') scrollToBottom();
     };
   } else {
     if(pillOuter) pillOuter.classList.remove('show');

tests/test_issue1360_streaming_scroll_hardening.py

@@ -0,0 +1,62 @@
+"""Regression tests for #1360: streaming must not re-pin user scroll."""
+
+from pathlib import Path
+
+REPO = Path(__file__).parent.parent
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+STYLE_CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
+
+
+def _extract_function(src: str, name: str) -> str:
+    marker = f"function {name}("
+    idx = src.find(marker)
+    assert idx != -1, f"{name} not found"
+    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(f"Could not extract {name}")
+
+
+def test_messages_scroller_disables_browser_scroll_anchoring():
+    assert "overflow-anchor:none" in STYLE_CSS, (
+        "#messages must disable browser scroll anchoring so tool/card inserts "
+        "cannot yank the transcript while the user reads earlier content."
+    )
+
+
+def test_scroll_repin_dead_zone_is_wider_for_mac_app_windows():
+    assert "clientHeight<250" in UI_JS, (
+        "The near-bottom re-pin threshold should be at least 250px so small "
+        "macOS app windows and trackpad momentum do not re-pin too eagerly."
+    )
+
+
+def test_queue_card_measurement_does_not_force_repin_during_streaming():
+    fn = _extract_function(UI_JS, "_renderQueueChips")
+    measurement_idx = fn.find("setTimeout(()=>")
+    assert measurement_idx != -1, "queue card measurement timeout not found"
+    measurement_block = fn[measurement_idx:measurement_idx + 500]
+
+    assert "S.activeStreamId" in measurement_block
+    assert "scrollIfPinned()" in measurement_block
+    assert "!S.activeStreamId" in measurement_block
+    assert "scrollToBottom()" in measurement_block
+    assert measurement_block.find("scrollIfPinned()") < measurement_block.find("scrollToBottom()")
+
+
+def test_queue_pill_click_does_not_force_repin_during_streaming():
+    fn = _extract_function(UI_JS, "_updateQueuePill")
+    click_idx = fn.find("pill.onclick=()=>")
+    assert click_idx != -1, "queue pill click handler not found"
+    click_block = fn[click_idx:click_idx + 700]
+
+    assert "S.activeStreamId" in click_block
+    assert "scrollIfPinned()" in click_block
+    assert "!S.activeStreamId" in click_block
+    assert "scrollToBottom()" in click_block
+    assert click_block.find("scrollIfPinned()") < click_block.find("scrollToBottom()")