Wonky mouse behavior when inside suckless tabbed (xembed)

As mentioned in regard to telegram-desktop in #16077;

When adding a window using particular Qt5 positioning API bits (we haven't narrowed it down, as seen in the issue) into suckless tabbed (xembed), there are major program-breaking issues with mouse alignment and behavior. In an investigation surrounding the telegram-desktop issue, we have narrowed it down to things that use absolute mouse positioning being mucked up, however more extensive testing would be necessary to determine the full scope of the incompatibility.

Qt 5.15.2, 5.6.2 and 5.3.2 are used by tdesktop specifically, not sure what the scope of the issue is.