[REG 5.12.0-5.12.1] closing QComboBox causes program to lose focus

With Qt 5.12.1 closing a combo box causes the current program to lose focus. I've created a video to demonstrate the behavior:

https://youtu.be/DwaPbZTQT5U

Effects can include no other window having focus afterwards or another window getting focus and being brought to the front.

How to reproduce: I'm using Qt's own "widgets/painting/basicdrawing" example, unmodified. Just compile it for Windows and run it.

I'm using the MXE project for cross-compiling both Qt itself as well as the "basicdrawing" example from Linux to Windows. The result is a static 64-bit binary.

The problem goes away when I compile and link against Qt 5.11.3 instead of 5.12.1. The problem occurs neither on Linux nor on macOS with 5.12.1,