I want to handle mousedown events, but not those which will trigger immediately later a contextmenu event.
As mousedown events are dispatched before contextmenu events, I can't "flag" the event somehow (e.g. by recording its timeStamp) in the contextmenu handler.
Of course, I can check event.ctrlKey in the mousedown handler, but that misses some other ways a context menu can be triggered (and, I assume, platform specific details). So how can I determine for sure if a contextmenu will follow?
You're right, that there are different ways to trigger the contextmenu event, however the issue, as you describe it, melts down to the case "mousedown and contextmenu".
In general a contextmenu event will follow a mousedown when the secondary mouse button was pressed. A special case for Mac we'll discuss later.
NB: The secondary button is usually the right one, when not configured otherwise. Left-handers e.g. will prefer the left mouse button. However, that's unimportant here, so lets stick to the configuration-independent term secondary button.
Now, you can check in your mousedown handler which button has been pressed, querying the button property of a MouseEvent object. The secondary button is represented by the number 2. So if MouseEvent.button == 2 in the mousedown handler, than don't execute that stuff you want to skip in case a contextmenu event was triggered.
Further, on Apple devices one can also employ the primary -- i.e. usually but not necessarily left -- mouse button while pressing the Control key. So the complete check in the mousedown handler on a MouseEvent object e is:
e.button == 2 || (e.button == 0 && e.ctrlKey)
If this is true, a contextmenu event will (right-click) or may (left-click + ctrl) follow. On Windows and Linux a left-click with Control will be misinterpreted as a click event after mouseup. Either hope that no ones come to that idea on Windows or Linux, or catch that case.
This should cover all mouse-related triggers for contextmenu on common operating systems, i.e. Windows, Linux and Mac.
As aarvinr pointed out in his comment, one can also employ the MouseEvent.buttons property for this purpose.