fix(webui): show new-message cue after unpinned mid-stream scroll restore

Address greptile review: the new S.activeStreamId restore branch omitted the
_maybeShowNewMessageScrollCue call that the preserveScroll and idle-unpin restore
branches both make. An unpinned reader whose position is restored mid-stream should
still see the new-message cue, so add it for consistency. Update the behavioral test
to assert the cue fires and the test_issue1690 source-lock for the new branch body.

static/ui.js

@@ -13440,6 +13440,7 @@ function _scrollAfterMessageRender(preserveScroll, scrollSnapshot){
     // the no-op. Pinned/tail-following readers keep scrollIfPinned() (correct live-follow).
     if(_messageUserUnpinned && scrollSnapshot){
       _restoreMessageScrollSnapshot(scrollSnapshot);
+      _maybeShowNewMessageScrollCue(scrollSnapshot);
       return;
     }
     scrollIfPinned();

tests/test_issue1690_scroll_completion.py

@@ -72,7 +72,7 @@ def test_render_messages_preserve_scroll_option_uses_user_pin_state_not_stream_l
     # browser's scrollTop clamp from the inner.innerHTML='' wipe, so it stranded the
     # reader at the top. Pinned/tail-following readers still take scrollIfPinned().
     assert "if(S.activeStreamId){" in scroll_helper
-    assert "if(_messageUserUnpinned && scrollSnapshot){\n      _restoreMessageScrollSnapshot(scrollSnapshot);\n      return;\n    }\n    scrollIfPinned();\n    return;\n  }" in scroll_helper
+    assert "if(_messageUserUnpinned && scrollSnapshot){\n      _restoreMessageScrollSnapshot(scrollSnapshot);\n      _maybeShowNewMessageScrollCue(scrollSnapshot);\n      return;\n    }\n    scrollIfPinned();\n    return;\n  }" in scroll_helper
 
 
 def test_cached_render_path_uses_same_scroll_policy_as_fresh_render():

tests/test_wipe_rows0_streaming_unpinned_restore.py

@@ -103,7 +103,7 @@ const performance = { now: performanceNow };
 function scrollIfPinned(){ calls.push('scrollIfPinned'); }
 function scrollToBottom(){ calls.push('scrollToBottom'); }
 function _followMessagesAfterDomReplace(){ calls.push('follow'); return true; }
-function _maybeShowNewMessageScrollCue(){ }
+function _maybeShowNewMessageScrollCue(){ calls.push('newMessageCue'); }
 // _restoreMessageScrollSnapshot is what the FIX must call for an unpinned reader.
 // Model its real effect: it moves scrollTop back toward the captured snapshot.top.
 function _restoreMessageScrollSnapshot(snap){
@@ -149,6 +149,11 @@ console.log(JSON.stringify({ calls, finalTop: el.scrollTop }));
         "after restore, scrollTop must return to the reader's pre-wipe position "
         f"(3000), not stay at the browser-clamped 0; got {m['finalTop']}"
     )
+    assert "newMessageCue" in m["calls"], (
+        "after restoring an unpinned reader mid-stream, the new-message cue must be "
+        "shown (consistent with the preserveScroll and idle-unpin restore branches); "
+        f"recorded calls={m['calls']!r}"
+    )
 
 
 def test_streaming_pinned_reader_still_follows_via_scrollifpinned():