Hi,
when using Mini Viewport, it is cropped from the bottom quite a bit, so one can't gauge, what is the camera really filming. 
When I make mini viewport smaller it is cropped even more, so It looks related. But the Mini Viewport is impossible to make larger .. it can be only enlarged more even when as separate window.

Put simply - main viewport is different than mini viewport. 

Any ideas? I guess this is most probably a bug.

Thanks,
Jirka