After updating to macOS 10.14.4 QtQuick based applications showing/hiding toplevel windows will cause all application windows to flicker.
This seems to be independent of the GPU (reported on "NVIDIA GeForce GTX 775M", "Intel Iris Plus Graphics 655").
There a more flicker problems that I've not yet been able to derive a test-case for but it seems all related to GL surface handling. None of this was a problem on 10.14.3.
The following trivial example demonstrates the effect: