QGraphicsProxyWidget does not draw transformations correctly (Buttons, checkboxes, etc...). This issue can be reproduced with the Qt5 Embedded Dialogs example. Attachments contains a screenshot which shows some of the issues.
modify main.cpp and customproxy.cpp by commenting out cache modes and/or changing them between NoCache (default), ItemCoordinateCache and DeviceCoordinateCache.
This is regression as the same example works fine with Qt 4.8.4