+1

Shouldn't this and the above function name start with small letter?

DBus methods and signals tend to start with an upper case. Though interface already consists of a mixture :/
Classes using Qt generated DBus adaptors have to match the interface they're implementing.

Wrote relevant matching kwin patch to close effects

Have you uploaded it? :-)

Arguably it won't help us with all cases because the clients would have to voluntarily participate in the protocol.

I wonder whether it's worth fixing this bug on X11 at all.

Arguably it won't help us with all cases because the clients would have to voluntarily participate in the protocol.

Yes. I'm not pretending it will.

I wonder whether it's worth fixing this bug on X11 at all.

There is a way to forcefully take a grab inside X internals.

The only way this is exposed publicly is via the shortcut key XF86Ungrab

setxkbmap -option grab:break_actions
xdotool key XF86Ungrab

It was meant as a dev tool so you could switch to a console to debug locks.
Using this is a horrific idea as it introduces a keyboard shortcut to circumvent the screen locker!!

There's no other public API, though I don't see why one couldn't be added.
The bigger problem is that the client side has no indication that it loses it's lock which leaves it in a corrupt state when you log in.
Adding that retroactively is the part I think is pragmatically impossible.

And before someone suggests injecting an escape key into the focused app, it might work, but we'd have to keep trying the lock until whatever kwin animation finishes which becomes a bit weird.