When registering native event filters on Windows - each mouse and keyboard event is duplicated. Othe the other hand, some events like WM_CHAR are "swallowed". I see the following output in logging for every event (left mouse button clicked, then one character typed):

WM_LBUTTONDOWN
WM_LBUTTONDOWN
WM_LBUTTONUP
WM_LBUTTONUP
WM_KEYDOWN
WM_KEYDOWN
WM_KEYUP
WM_KEYUP

While plain C WinAPI message loop logging results in:

WM_LBUTTONDOWN
WM_LBUTTONUP
WM_KEYDOWN
WM_CHAR
WM_KEYUP

The test project (find attached) is a basic QML project using Visual Studio 2015 32-bit for building. It creates single blank ApplicationWindow and registers object of class inheriting from QAbstractNativeEventFilter using QApplication::installNativeEventFilter.

The issue is serious since we have some 3rd party components that depend on correct handling of native windows events and the current message loop seems messed up...