test(desktop): wait for initial transcript tail

apps/desktop/e2e/transcript-scroll.spec.ts

@@ -634,6 +634,25 @@ test('history asked for at the very top of the scroller still lands above the re
     .getAttribute('data-transcript-turn-id');
   const firstBefore = await firstLoadedTurn();
 
+  // The fixture is ready when the transcript exists, before its initial tail
+  // positioning necessarily completes. Writing zero while it is still at zero
+  // is a no-op, so no reader scroll event asks for earlier history. Require one
+  // geometry sample to prove both that the initial pin moved and where it
+  // landed before exercising the real move to the top.
+  await expect.poll(async () => {
+    const metrics = await scrollMetrics(page);
+    return {
+      positionedAwayFromStart: metrics.scrollTop > 0,
+      settledAtTail: metrics.distance <= 4,
+      metrics,
+    };
+  }, {
+    message: 'the initial transcript tail positioning settles',
+  }).toMatchObject({
+    positionedAwayFromStart: true,
+    settledAtTail: true,
+  });
+
   // The one position where the browser declines to anchor, and the one the
   // wheel-to-load path puts the reader in.
   await page.evaluate((selector) => {