fix(webui): compact spacing between auxiliary trace rows

Thinking and Used tools are both auxiliary trace rows, but the thread list
was applying the same large gap used between full chat turns. That made
alternating Thinking / Used tools sequences look uneven and too airy.

Move row spacing from a fixed flex gap to per-row margins: full chat turns
keep mt-5, while consecutive auxiliary rows use mt-2. Add coverage for
Thinking -> Used tools -> Thinking spacing.

Co-authored-by: Cursor <cursoragent@cursor.com>

webui/src/components/thread/ThreadMessages.tsx

@@ -1,4 +1,5 @@
 import { MessageBubble } from "@/components/MessageBubble";
+import { cn } from "@/lib/utils";
 import type { UIMessage } from "@/lib/types";
 
 interface ThreadMessagesProps {
@@ -7,10 +8,30 @@ interface ThreadMessagesProps {
 
 export function ThreadMessages({ messages }: ThreadMessagesProps) {
   return (
-    <div className="flex w-full flex-col gap-5">
-      {messages.map((message) => (
-        <MessageBubble key={message.id} message={message} />
-      ))}
+    <div className="flex w-full flex-col">
+      {messages.map((message, index) => {
+        const prev = messages[index - 1];
+        const compact = isAuxiliaryRow(message) && prev && isAuxiliaryRow(prev);
+        return (
+          <div
+            key={message.id}
+            className={cn(index > 0 && (compact ? "mt-2" : "mt-5"))}
+          >
+            <MessageBubble message={message} />
+          </div>
+        );
+      })}
     </div>
   );
 }
+
+function isAuxiliaryRow(message: UIMessage): boolean {
+  return (
+    message.kind === "trace"
+    || (
+      message.role === "assistant"
+      && message.content.trim().length === 0
+      && (!!message.reasoning || !!message.reasoningStreaming)
+    )
+  );
+}