Application clickthrough with trackpad tap sends mousedown but no mouseup

To reproduce, open a page with the simplebrowser example application, then open developer tools and run in the console:

document.addEventListener("mousedown", function(event) { console.log(event); }, true)
document.addEventListener("mouseup", function(event) { console.log(event); }, true)

Then, try to test application click-through. With the developer tools window focused, use the trackpad to make a 'tap' click on the web page in the other window. (Note that you may need to enable 'Tap to Click' in the macOS Trackpad preferences page if you haven't done so.)

You should see a mousedown event emitted in the developer console, but no accompanying mouseup.

This can confuse some web applications; e.g. the mousedown might be interpreted as the start of a drag event, and since no mouseup event was observed the behavior the web application will erroneously believe the user is attempting to perform a mouse drag.

At the Qt level, if I install an event filter on qApp that just logs Qt mouse events, for normal clicks I see a pair of MousePress / MouseRelease events; for some reason, on the click-through trackpad tap I see a pair of MousePress events but just one single MouseRelease event. I suspect this missing MouseRelease event is the source of the issue.