When Qt runs in process with low integrity level (https://msdn.microsoft.com/en-us/library/bb625963.aspx) and place QWindow to HWND from less restricted process, it could generate wrong mouse leave event on mouse button up, even if mouse still over window.
Example of such situation is when Qt is used in web browser plugin. Most browsers runs plug-ins in separate processes, but Internet Explorer go further and runs plug-ins in low integrity level process (i.e. restricted).
To see the problem, you could:
1) Install .msi from https://github.com/RSATom/WebChimera/releases/tag/v.0.2.3
2) Open http://rsatom.github.io/WebChimera/demos/basic/basic_1.htm with Intenet Explorer
3) Move mouse over browser plugin window - play/pause button will show at bottom.
4) If you click on button - it will hide (since QuickView will get wrong "mouse leave" event).
5) If you open same link on Firefox - and click on button - it will not hide.
Patch for this issue already created: https://codereview.qt-project.org/#/c/104523/ (I will add reference to it at evening) - but needs approvement.