--- a/widget/tests/test_sizemode_events.xhtml
+++ b/widget/tests/test_sizemode_events.xhtml
@@ -19,18 +19,19 @@
 <script class="testbody">
 <![CDATA[
 
 const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
 let gWindow = null;
 
 const kIsLinux = navigator.platform.includes("Lin");
-// On Linux sizemode changes might be async.
-const kAsyncChanges = kIsLinux;
+const kIsMacOS = navigator.platform.includes("Mac");
+// On Linux and macOS sizemode changes might be async.
+const kAsyncChanges = kIsLinux || kIsMacOS;
 
 let gSizeModeDidChange = false;
 let gSizeModeDidChangeTo = 0;
 
 function sizemodeChanged(e) {
   gSizeModeDidChange = true;
   gSizeModeDidChangeTo = gWindow.windowState;
 }