fix: remount post-steer stream rows

ui/src/e2e/chat-flow.active-run-follow-ups.e2e.test.ts

@@ -277,6 +277,10 @@ suite.define(() => {
         result: "process complete",
         toolCallId: "callProcess",
       });
+      const workingRowKey = await page
+        .locator("[data-virtual-row-key^='stream-run:']")
+        .last()
+        .getAttribute("data-virtual-row-key");
       const finalText = Array.from(
         { length: 18 },
         (_, index) =>
@@ -302,6 +306,7 @@ suite.define(() => {
         "xpath=ancestor::div[contains(@class, 'chat-virtual-row')]",
       );
       await streamingRow.waitFor();
+      expect(await streamingRow.getAttribute("data-virtual-row-key")).not.toBe(workingRowKey);
       const steerBubble = page.locator(".chat-group.user", { hasText: steerText }).last();
       const [steerBounds, streamingBounds] = await Promise.all([
         steerBubble.boundingBox(),

ui/src/pages/chat/chat-thread-build.ts

@@ -714,7 +714,9 @@ export function buildChatItems(props: BuildChatItemsProps): Array<ChatItem | Mes
       const liveProgress = resolveProgress();
       const liveStreamItem: ChatItem = {
         kind: "stream",
-        key: liveProgress.key,
+        key: latestBoundaryRunId
+          ? `${liveProgress.key}:after:${latestBoundaryRunId}`
+          : liveProgress.key,
         text: visibleText,
         startedAt: timestampAfterVisibleItems(items, props.streamStartedAt ?? Date.now()),
         isStreaming: true,