Bug 1662313: Add a wait on a selectionchange event to test_resizers_resizing_elements to prevent race condition in test; r=masayuki DONTBUILD

Wait for `selectionchange` to ensure that input events have been processed
before verifying a target's bounds.

Differential Revision: https://phabricator.services.mozilla.com/D88936

editor/libeditor/tests/test_resizers_resizing_elements.html

@@ -147,8 +147,12 @@ SimpleTest.waitForFocus(async () => {
       // Move the mouse delta more pixels to the same direction to make sure that the
       // resize operation has stopped.
       synthesizeMouse(target, basePosX + deltaX * 2, basePosY + deltaY * 2, {type: "mousemove"});
+
       // Click outside of the editor to hide the resizers
+      ok(true, "waiting selectionchange to select outside the target element");
+      let promiseSelectionExitEvent = waitForSelectionChange();
       synthesizeMouseAtCenter(outOfEditor, {});
+      await promiseSelectionExitEvent;
 
       // Get the new dimensions for the target
       // XXX I don't know why we need 2px margin to check this on Android.