If an application contains QOpenGLWidget then there is flicker before rendering content. For example, when opening a combo box or showing a tooltip the popup first flickers black and then shows the content.

This was tested on Windows 10 with desktop OpenGL. The problem is not reproducible with Qt Quick applications nor with basic widget applications. I didn't test all possible Qt versions, but the problem appeared sometime between 5.11.0 and 5.11.2.

The problem can be reproduced with Qt's OpenGL Widget example. The flicker might be seen already when opening the main window, but it is clearer when hovering on top of 'Update Interval' field and the tooltip is shown.