fix: prevent agent progress layout jitter

The previous popLayout stabilization kept margin-based spacing, so exiting rows temporarily added 6px to bounded Agent lists.

Use flex gaps and a geometry regression check so exited rows no longer change container height.

tests/browser/agent-streaming-stability.e2e.ts

@@ -4,6 +4,7 @@ import { expect, test } from "playwright/test";
 
 const STREAM_CHUNK_SIZE = 18;
 const STREAM_CHUNK_DELAY_MS = 20;
+const PROGRESS_GEOMETRY_TOLERANCE_PX = 0.1;
 
 let streamServer: Server;
 let streamServerUrl: string;
@@ -267,19 +268,54 @@ test("stream completion and the plan card remain positionally stable", async ({
   const liveProgress = page.getByRole("list", { name: "最近执行进度" });
   await expect(liveProgress.locator("li")).toHaveCount(3);
   await expect(liveProgress.getByText("正在分析运行上下文", { exact: true })).toBeVisible();
+  await expect(liveProgress).toHaveCSS("height", "156px");
+  await expect.poll(() => liveProgress.evaluate((list) => list.style.height)).toBe("auto");
 
   await page.evaluate(() => {
     const state = window as typeof window & {
       __agentMinVisibleProgressRows?: number;
+      __agentProgressGeometry?: {
+        maxContainerHeight: number;
+        maxListHeight: number;
+        minContainerHeight: number;
+        minListHeight: number;
+      };
       __agentProgressWindowSampleDone?: boolean;
     };
     const startedAt = performance.now();
     state.__agentMinVisibleProgressRows = Number.POSITIVE_INFINITY;
+    state.__agentProgressGeometry = {
+      maxContainerHeight: Number.NEGATIVE_INFINITY,
+      maxListHeight: Number.NEGATIVE_INFINITY,
+      minContainerHeight: Number.POSITIVE_INFINITY,
+      minListHeight: Number.POSITIVE_INFINITY
+    };
     state.__agentProgressWindowSampleDone = false;
 
     const sample = () => {
-      const rows = document.querySelectorAll('ol[aria-label="最近执行进度"] > li').length;
+      const list = document.querySelector<HTMLOListElement>('ol[aria-label="最近执行进度"]');
+      const rows = list?.children.length ?? 0;
       state.__agentMinVisibleProgressRows = Math.min(state.__agentMinVisibleProgressRows ?? rows, rows);
+      if (list && state.__agentProgressGeometry) {
+        const listHeight = list.getBoundingClientRect().height;
+        const containerHeight = list.parentElement?.getBoundingClientRect().height ?? 0;
+        state.__agentProgressGeometry.minListHeight = Math.min(
+          state.__agentProgressGeometry.minListHeight,
+          listHeight
+        );
+        state.__agentProgressGeometry.maxListHeight = Math.max(
+          state.__agentProgressGeometry.maxListHeight,
+          listHeight
+        );
+        state.__agentProgressGeometry.minContainerHeight = Math.min(
+          state.__agentProgressGeometry.minContainerHeight,
+          containerHeight
+        );
+        state.__agentProgressGeometry.maxContainerHeight = Math.max(
+          state.__agentProgressGeometry.maxContainerHeight,
+          containerHeight
+        );
+      }
       if (performance.now() - startedAt < 400) {
         requestAnimationFrame(sample);
       } else {
@@ -303,6 +339,23 @@ test("stream completion and the plan card remain positionally stable", async ({
       (window as typeof window & { __agentMinVisibleProgressRows?: number }).__agentMinVisibleProgressRows
     )
   ).toBeGreaterThanOrEqual(3);
+  const progressGeometry = await page.evaluate(() =>
+    (window as typeof window & {
+      __agentProgressGeometry?: {
+        maxContainerHeight: number;
+        maxListHeight: number;
+        minContainerHeight: number;
+        minListHeight: number;
+      };
+    }).__agentProgressGeometry
+  );
+  expect(progressGeometry).toBeDefined();
+  expect(progressGeometry!.maxListHeight - progressGeometry!.minListHeight).toBeLessThanOrEqual(
+    PROGRESS_GEOMETRY_TOLERANCE_PX
+  );
+  expect(progressGeometry!.maxContainerHeight - progressGeometry!.minContainerHeight).toBeLessThanOrEqual(
+    PROGRESS_GEOMETRY_TOLERANCE_PX
+  );
 
   const liveProgressRowHeights = await liveProgress.locator("li").evaluateAll((rows) =>
     rows.map((row) => row.getBoundingClientRect().height)