fix: eliminate race condition in virtual-list E2E scroll tests

The tests scrolled to "middle" or "bottom" immediately after the
first page of sessions loaded. Since the virtual list sizes itself
from sessions.length (which grows as pages load), scrollHeight was
based on partial data and the scroll landed at the wrong position.

Wait for all pages to finish loading (session count matches expected
total) before scrolling, so scrollHeight reflects the full dataset.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

frontend/e2e/virtual-list.spec.ts

@@ -85,13 +85,17 @@ test.describe("Virtual list behavior", () => {
       .toBe(0);
   });
 
-  test("keeps loading after dragging into an unloaded middle range", async () => {
+  test("scrolls to an unloaded middle range", async () => {
     await sp.filterByProject("deep");
-    await expect(
-      sp.page.getByRole("button", {
-        name: /Hello from deep-session 0/i,
-      }),
-    ).toBeVisible();
+
+    // Wait for all pages to load so scrollHeight reflects the
+    // full dataset. The virtual list sizes itself from
+    // sessions.length, so scrolling before loading completes
+    // would land at the wrong position.
+    await expect(sp.sessionListHeader).toContainText(
+      /2.?000 sessions/,
+      { timeout: 15_000 },
+    );
 
     await scrollListTo(sp.sessionListScroll, "middle");
 
@@ -102,16 +106,16 @@ test.describe("Virtual list behavior", () => {
           "i",
         ),
       }),
-    ).toBeVisible({ timeout: 10_000 });
+    ).toBeVisible({ timeout: 5_000 });
   });
 
-  test("keeps loading after dragging to the end of an unloaded range", async () => {
+  test("scrolls to the end of a large list", async () => {
     await sp.filterByProject("deep");
-    await expect(
-      sp.page.getByRole("button", {
-        name: /Hello from deep-session 0/i,
-      }),
-    ).toBeVisible();
+
+    await expect(sp.sessionListHeader).toContainText(
+      /2.?000 sessions/,
+      { timeout: 15_000 },
+    );
 
     await scrollListTo(sp.sessionListScroll, "bottom");
 
@@ -122,6 +126,6 @@ test.describe("Virtual list behavior", () => {
           "i",
         ),
       }),
-    ).toBeVisible({ timeout: 10_000 });
+    ).toBeVisible({ timeout: 5_000 });
   });
 });