Bug 1880529 - Requst animation frame at start of browser_test_resize. r=sfoster

Differential Revision: https://phabricator.services.mozilla.com/D208036

browser/components/screenshots/tests/browser/browser_test_resize.js

@@ -12,6 +12,8 @@ add_task(async function test_window_resize() {
       url: RESIZE_TEST_PAGE,
     },
     async browser => {
+      await new Promise(r => window.requestAnimationFrame(r));
+
       let helper = new ScreenshotsHelper(browser);
       await helper.resizeContentWindow(windowWidth, window.outerHeight);
       const originalContentDimensions = await helper.getContentDimensions();
@@ -61,6 +63,8 @@ add_task(async function test_window_resize_vertical_writing_mode() {
         content.document.documentElement.style = "writing-mode: vertical-lr;";
       });
 
+      await new Promise(r => window.requestAnimationFrame(r));
+
       let helper = new ScreenshotsHelper(browser);
       await helper.resizeContentWindow(windowWidth, window.outerHeight);
       const originalContentDimensions = await helper.getContentDimensions();