fix: preserve sidebar scrolling while streaming

docs/pr-media/1784/sidebar-scroll-qa.json

@@ -0,0 +1,25 @@
+{
+  "issue": 1784,
+  "commit_under_test": "9875967",
+  "fixture": "Synthetic 180-row session sidebar with active sid_0 streaming and long chat pane content.",
+  "pre_fix_observation": {
+    "steps": [
+      "Set _scrollPinned=true with #messages at scrollTop 0 in a long chat fixture.",
+      "Dispatch a wheel gesture on the active sidebar session row.",
+      "Call scrollIfPinned() to mimic the next streaming token render."
+    ],
+    "result": "#messages jumped from scrollTop 0 to 3073 immediately after the sidebar wheel gesture, showing the chat auto-scroll path fought non-chat scroll intent."
+  },
+  "post_fix_observation": {
+    "steps": [
+      "Repeat the same fixture and sidebar wheel gesture after the fix.",
+      "Call scrollIfPinned() immediately, then again after the 350ms non-chat intent guard expires."
+    ],
+    "result": {
+      "afterSidebarWheel": 0,
+      "afterIntentExpires": 2992,
+      "sessionListCss": "overscroll-behavior-y: contain; touch-action: pan-y"
+    },
+    "meaning": "A sidebar wheel/touch scroll intent now suppresses only the immediate chat-pane auto-scroll write, leaving the sidebar gesture free while streaming continues."
+  }
+}

static/style.css

@@ -321,7 +321,7 @@
   .sidebar-section{padding:14px 14px 8px;}
   .new-chat-btn{width:100%;padding:9px 12px;border-radius:9px;background:var(--accent-bg);border:1px solid var(--accent-bg-strong);color:var(--accent-text);font-size:13px;cursor:pointer;display:flex;align-items:center;gap:8px;transition:all .15s;margin-bottom:8px;font-weight:500;}
   .new-chat-btn:hover{background:var(--accent-bg-strong);border-color:var(--accent);}
-  .session-list{flex:1;overflow-y:auto;padding:0 8px 8px;min-height:0;}
+  .session-list{flex:1;overflow-y:auto;padding:0 8px 8px;min-height:0;overscroll-behavior-y:contain;touch-action:pan-y;}
   .sidebar-search{position:relative;padding:8px 12px;flex-shrink:0;}
   .sidebar-search input{width:100%;background:var(--bg);border:1px solid var(--border);border-radius:8px;color:var(--text);padding:7px 10px 7px 32px;font-size:13px;outline:none;transition:border-color .15s,box-shadow .15s,background .15s;box-sizing:border-box;}
   .sidebar-search input:focus{border-color:var(--accent);box-shadow:0 0 0 3px var(--accent-bg);}

static/ui.js

@@ -1472,6 +1472,25 @@ let _scrollPinned=true;
 let _programmaticScroll=false;
 let _nearBottomCount=0;
 let _lastScrollTop=null;
+let _lastNonMessageScrollIntentMs=0;
+const NON_MESSAGE_SCROLL_INTENT_SUPPRESS_MS=350;
+function _recordNonMessageScrollIntent(e){
+  const el=document.getElementById('messages');
+  const target=e&&e.target;
+  if(!el||!target) return;
+  // Streaming token renders should keep pinning the chat only while the user is
+  // actually interacting with the chat pane. A wheel/touch gesture over the
+  // session sidebar (or another independent pane) must not be immediately fought
+  // by scrollIfPinned() writing #messages.scrollTop on the next token (#1784).
+  if(!el.contains(target)) _lastNonMessageScrollIntentMs=performance.now();
+}
+function _recentNonMessageScrollIntent(){
+  return performance.now()-_lastNonMessageScrollIntentMs<NON_MESSAGE_SCROLL_INTENT_SUPPRESS_MS;
+}
+if(typeof document!=='undefined'){
+  document.addEventListener('wheel',_recordNonMessageScrollIntent,{capture:true,passive:true});
+  document.addEventListener('touchmove',_recordNonMessageScrollIntent,{capture:true,passive:true});
+}
 // Reset hook for session-switch — called from sessions.js loadSession() to
 // prevent the new chat's first scroll comparing against the previous chat's
 // scrollTop (Opus stage-302 SHOULD-FIX, #1731 follow-up).
@@ -1778,6 +1797,7 @@ document.addEventListener('DOMContentLoaded',function(){
 
 function scrollIfPinned(){
   if(!_scrollPinned) return;
+  if(_recentNonMessageScrollIntent()) return;
   const el=$('messages');
   if(el){_programmaticScroll=true;el.scrollTop=el.scrollHeight;setTimeout(()=>{_programmaticScroll=false;},0);}
 }

tests/test_streaming_sidebar_scroll.py

@@ -0,0 +1,47 @@
+"""Regression tests for #1784: sidebar scroll remains independent while streaming."""
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+STYLE_CSS = (ROOT / "static" / "style.css").read_text(encoding="utf-8")
+
+
+def _extract_fn(src: str, name: str) -> str:
+    marker = f"function {name}"
+    start = src.find(marker)
+    assert start >= 0, f"{name} not found"
+    brace = src.find("{", start)
+    assert brace >= 0, f"{name} body not found"
+    depth = 0
+    for i in range(brace, len(src)):
+        ch = src[i]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[start : i + 1]
+    raise AssertionError(f"{name} body did not close")
+
+
+def test_sidebar_wheel_intent_is_recorded_passively():
+    """A sidebar wheel gesture must not be swallowed or ignored during streaming."""
+    assert "_recordNonMessageScrollIntent" in UI_JS
+    assert "document.addEventListener('wheel',_recordNonMessageScrollIntent" in UI_JS
+    assert "{capture:true,passive:true}" in UI_JS
+    assert "!el.contains(target)" in UI_JS
+    assert "_lastNonMessageScrollIntentMs=performance.now()" in UI_JS
+
+
+def test_scroll_if_pinned_skips_during_recent_non_message_scroll():
+    """Token rendering must not force-scroll #messages while the sidebar is being scrolled."""
+    fn = _extract_fn(UI_JS, "scrollIfPinned")
+    assert "_recentNonMessageScrollIntent()" in fn
+    guard_index = fn.find("_recentNonMessageScrollIntent()")
+    write_index = fn.find("scrollTop=el.scrollHeight")
+    assert guard_index >= 0 and write_index >= 0 and guard_index < write_index
+
+
+def test_session_list_has_its_own_scroll_boundary():
+    """The session list is its own scroll surface, not chained to the chat/body scroller."""
+    assert ".session-list{flex:1;overflow-y:auto;padding:0 8px 8px;min-height:0;overscroll-behavior-y:contain;touch-action:pan-y;}" in STYLE_CSS