fix(desktop): keep buffered timeline catch-up reachable

A frozen logical tail can report physical bottom before buffered live rows are released. Keep the existing catch-up action visible while pending rows remain so a shared thread message can enter the parent timeline after closing the thread. Preserve scroll anchoring and explicit catch-up behavior.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Signed-off-by: Logan Johnson <loganj@squareup.com>

Author: loganj

desktop/src/features/messages/ui/MessageTimeline.tsx

@@ -875,7 +875,9 @@ const MessageTimelineBase = React.forwardRef<
           )}
         </div>
 
-        {!isAtBottom ? (
+        {/* A frozen tail can be physically at bottom while live rows are still
+            buffered. Keep the release action reachable in that state. */}
+        {!isAtBottom || bufferedTimeline.pendingCount > 0 ? (
           <div
             className={cn(
               "pointer-events-none absolute inset-x-0 bottom-4 z-50 flex justify-center px-4",