Missing mouse release event for middle button for inactive windows

Three related symptoms:

• When using the middle mouse button on an inactive window, only the press event is delivered and not the release event (expected behavior is to also get a release event).
• For each next middle mouse button click, a single mouse move event is delivered (expected behavior is to get a press and a release event instead).
• When making the window active, the delivered mouse move events suggest that the middle mouse button is being held down, even if it isn't.

I'm attaching a main.cpp file that allows reproducing the issue.

In my application I've implemented viewport panning with the middle mouse button, as is common in graphical tools like Inkscape and GIMP. Recently, somebody on OS X reported problems with the behavior when trying to pan the view when the application is not the active window, which I've found to be due to the above issue.