Bug 1708697 - Setup a scroll event listener before triggering an autoscroll operation. r=tnikkel

Depends on D128953

Differential Revision: https://phabricator.services.mozilla.com/D128954

gfx/layers/apz/test/mochitest/browser_test_autoscrolling_in_extension_popup_window.js

@@ -115,16 +115,6 @@ add_task(async () => {
     }
   );
 
-  // Start autoscrolling.
-  ok(
-    browserForPopup.browsingContext.startApzAutoscroll(
-      screenX + 100,
-      screenY + 50,
-      viewId,
-      presShellId
-    )
-  );
-
   const scrollEventPromise = SpecialPowers.spawn(
     browserForPopup,
     [],
@@ -142,6 +132,16 @@ add_task(async () => {
     }
   );
 
+  // Start autoscrolling.
+  ok(
+    browserForPopup.browsingContext.startApzAutoscroll(
+      screenX + 100,
+      screenY + 50,
+      viewId,
+      presShellId
+    )
+  );
+
   // Send sequential mousemove events to cause autoscrolling.
   for (let i = 0; i < 10; i++) {
     await promiseNativeMouseEventWithAPZ({