qtwebengine breaks on X11 if a QGLWidget or a widget with native window is in the same QMainWindow

Consider a QMainWindow, that has a QStackedWidget as child, which has a QWebEngineView as child.

This works perfectly fine, until you call winId() on the QStackedWidget. Then the QWebEngineView renders nothing. The same can be observed when creating a separate QWidget and make it native; various attempt to get it to work by containing the QWebEngineView or the native widget in other widgets (native or not) failed. It also fails when creating a QGLWidget as a direct or indirect sibling.

Usually, Qt will print this message once or twice to the terminal when this happens: "QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined"

Setting QT_USE_NATIVE_WINDOWS=1 when starting the program makes it work slightly better. The window isn't black anymore most of the time, but it can turn black temporarily, and might flicker or not repaint. (Still unusable.)

I'm assuming this is a bug. I didn't test on windows or OSX, although there's an unreliable report that it might work on OSX.

I have no minimal test case. If one is absolutely needed, please say so and I can write one. Though I assume that it doesn't need much to reproduce.