WASM: indifferent behaviour for mouse clicks

After updating our app development to 6.5.0-beta1, webassembly mouse input seems to be messed up: there's partly no reaction on mouse clicks, while hover or wheel scrolling works.

I've spent a whole day with trying to find some patterns without success yet. I can comment parts in the app that even don't have a mouse handler, and a test button starts working. Trying to distill that to a small example doesn't show any problems, though. It's quite frustrating and sometimes feels like it's dependent on a button position on the screen, sometimes it seems to be related to obviously unrelated stuff. Even if I try to find any possibly offending elements in gammaray on the desktop, there is nothing.

There is absolutely no problem on other platforms. It just works like it did in 6.4.1.

I just want to let you know that there's something broken that renders the complete application unusable. If you have any idea what I can try to reproduce it - based on what changed - please let me know. The only thing left here would be bisecting Qt to find the commit which is introducing this behaviour, but as I already lost much time, I need to do productive things first.