While creating a layout test for bug 168424 I was not able to make it pass on the MAC WK1. If I use the eventSender.mouseUp(0, ['altKey']); in the test then the event is created in Tools/DumpRenderTree/mac/EventSendingController.mm in the mouseUp() function. In the modifierFlags function (in the same module) the altKey modifier is recognized and set. But then in Source/WebKit/mac/WebView/WebView.mm in the applicationFlags() function where we get the current event - [[NSApp currentEvent] modifierFlags] - it is 0 - no modifier set. The same applies for other modifiers as well. If I try the test in the MiniBrowser, then it works as expected.