the QDropEvents return the wrong value for "buttons":

• expected behaviour: state of the buttons after the drop event has been handled
• current behaviour: state of the buttons before the drop event has been handled

the expected behaviour is consistent across macos/win32/xcb.

can be reproduced with this manual test:
https://codereview.qt-project.org/c/qt/qtbase/+/681709

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from my initial reading of the code, the culprit is that it relies on QGuiApplication::mouseButtons()