QtWebengine: Overlay positions miscalculated on rotated windows

Overlay boxes such as an expanded dropdown list of the HTML <select> tag are opening up at miscalculated screen positions instead of being attached at the root element when a rotation != 0deg is applied.

The location of the misplaced overlays is not random, we believe the overlays are opening up at the screen position of the unrotated root element. For instance, the dropdown list of a select element in the upper left corner of the screen will open up at the opposite corner when rotating the screen upside down.

However, the overlay seems to be aware of the new screen dimensions as it makes sure not to expand outside of the visible screen. And the overlay also seems to be aware of the new orientation because the elements of the dropdown list are always correctly oriented and readable (that is, the text of the elements is never displayed upside down).

This bug can be reproduced with yocto/warrior and yocto/dunfell. That would be 5.12.9 and 5.14.2, respectively.

We are using the iio-sensor-proxy for converting sensor data from an i2c attached accel. sensor to logical orientation events on dbus. The screenshot attached shows the behaviour of QtWenengine on the target when receiving the "LeftUp" orientation event.