Window content does not update until window gets resized

Starting from Qt 5.12 I found that the entire window content will not visually update until the window gets resized. (Everything was fine in Qt 5.11)

The attached example opens a QtQuick ApplicationWindow containing a CheckBox. Clicking the checkbox triggers the correct event and everything, but the visual appearance does not change at all. To force redraw, you can resize the window by mouse.

I reproduced this on two different Ubuntu 18.04 boxes, one with Intel Graphics one with Nvidia.

A possible workaround is to manually set QSG_RENDER_LOOP=windows

The two other values "basic" and "threaded" are affected by this issue, only "windows" appears to work. But "basic" is the default.