QWindow activated in testcase crashing Metacity

Having QWindow instantiated and activated in a testcase is able to crash Metacity. Appears to be root cause for QTBUG-24462.

What happens is Metacity printing:
Bug in window manager: Unexpected X error: BadWindow (invalid Window parameter) serial 749 error_code 3 request_code 2 minor_code 0)

With other window manager this worked ok, but did output a bit different X errors either from Xephyr or the window manager, such as:

error 3 request 20 minor 0 serial 213
error 3 request 15 minor 0 serial 214
error 9 request 150 minor 1 serial 374

I'm not sure if this is only Metacity bug or also Qt/XCB backend, X errors would hint that XCB is doing something wrong. Be it either one, Qt CI is using Metacity for running the tests so this is affecting development.

Attached testcase is able to trigger this in a few iterations here.