Windows: Wrong mouse buttons state after drag and drop

In our application we need some idle processing in the background. We are using QApplication::mouseButtons() as one of our checks in the Idle timer event (see attached example).
The problem is that this method returns wrong state after drag-n-drop operations on the Windows. Please run attached example under debugger to see qDebug() output.
There are three kinds of DnD possible:
a) an internal DnD between two windows of test app;
b) an external DnD from test app into the Windows Explorer;
c) an external DnD from Explorer into the test app;
Note that all kinds of DnD can be canceled with Escape key.
Bugs:
1. The mouse button state of QApplication stays wrong after cases b) and c)
2. Alt modifier key is interpreted as XButton1 during DnD