fix(replay-ui): skip synthetic zero-bounds root in deviceSpaceOf

The iOS hierarchy prepends a zero-bounds node before the real root
window, so elements[0] returned undefined and the overlay fell back to
the screenshot's pixel size. Take the first element with positive
extent instead; pre-order puts the root window before any content.
Verified against a real iOS trace in the replay UI.

replay-ui/src/__tests__/device-space.test.ts

@@ -33,4 +33,15 @@ describe("deviceSpaceOf", () => {
     expect(deviceSpaceOf(hierarchyWithRoot(393, 0))).toBeUndefined();
     expect(deviceSpaceOf(hierarchyWithRoot(-1, -1))).toBeUndefined();
   });
+
+  it("skips the iOS synthetic zero-bounds root", () => {
+    const hierarchy: Hierarchy = {
+      elements: [
+        { bounds: { left: 0, top: 0, right: 0, bottom: 0 } },
+        { bounds: { left: 0, top: 0, right: 402, bottom: 874 } },
+        { bounds: { left: 20, top: 100, right: 380, bottom: 150 } },
+      ],
+    };
+    expect(deviceSpaceOf(hierarchy)).toEqual({ width: 402, height: 874 });
+  });
 });

replay-ui/src/lib/device-space.ts

@@ -3,14 +3,18 @@ import type { Hierarchy } from "../types";
 // Tap points and resolved bounds in the trace share the hierarchy root's
 // coordinate space (iOS points, Android pixels, web CSS px). Screenshots may
 // be scaled (iOS 3x, web DPR>1), so the overlay viewBox must come from the
-// root bounds, not the image's natural pixel size.
+// root bounds, not the image's natural pixel size. Elements are in pre-order;
+// the first one with positive extent is the root window (iOS prepends a
+// synthetic zero-bounds node, so plain elements[0] is not enough).
 export function deviceSpaceOf(
   hierarchy?: Hierarchy,
 ): { width: number; height: number } | undefined {
-  const root = hierarchy?.elements?.[0];
-  if (!root) return undefined;
-  const width = root.bounds.right;
-  const height = root.bounds.bottom;
-  if (!(width > 0) || !(height > 0)) return undefined;
+  for (const element of hierarchy?.elements ?? []) {
+    const width = element.bounds.right;
+    const height = element.bounds.bottom;
+    if (width > 0 && height > 0) {
       return { width, height };
     }
+  }
+  return undefined;
+}