I have a weired sympton with a fresh Windos 10 guest OS under VirtualBox 5.1.28 under Ubutunu-MATE 16.04:
I can't move the window at all, when
[x] Auto Capture Keyboard is on.
Neither dragging the outer windows borders (resize), nor the titlebar (indeed just move the virtuabl box. So this is not about auto-resize on or off...). If I deactivate the option, things work fine again. (I do not even understand the relation). – sounds like a virtual box bug? Can't reproduce the opposite, but I have been working with virtual box (and Win7 guests) for long, never encountered this until 2 weeks ago.
But then, common shortcuts I have defined in host and guest, don't work on the guest but rather the host (i.e. WIN-R to get a run programm...). So, of course capturing is desirable.