There's a bunch of finger pointing with Qt saying Windows' OpenGL
implementation is broken. There is a workaround added in Qt 5.6.
http://doc.qt.io/qt-5/windows-issues.html
Fullscreen on Windows will then add a 1px border to Qt by default. This
can be fixed by patching Qt.
https://bugreports.qt.io/browse/QTBUG-41309