Viewport does not align with window boundaries

For some reason, the screen area OpenGL draws into does not align with the window area. There is an offset of about [-11, 11] pixels (seen from the top left), hence the black borders on the top and right (I changed the clear color to red in `ShapeDemo`). `OpenGLWindow::geometry()` reports `0, 0, 1280, 720` relative to the main window (which is correct, in theory).


Using Qt 5.9.1 on Windows 10 x64 with Visual Studio 2017.

Start by reproducing the offset on Windows 10 with Qt 5.9.1 and Visual Studio 2017, using ShapeDemo and comparing OpenGLWindow::geometry() with the actual rendered area. Inspect the window and viewport boundary handling; done means the OpenGL content aligns with the window without the reported black borders.

Written by the indexing model from the issue text.