Update: I’ve just tried an opencv-4.4.0 build with QT and OpenGL. No problem for building, but passing a GpuMat to imshow for a cv::WINDOW_OPENGL window doesn’t work.
I’ve not been able to configure OpenGL support with GTK3, but it builds fine with GTK2, and this works as before.
If you’re having problems with QT, you may try that instead:
-D WITH_GTK=ON -D WITH_GTK_2_X=ON
Sorry for the bad advice and the wasted time.