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.
I find the same thing when imwheel is running, the utility that allows change to the mouse wheel. Holding down the Right-Control dows allow VirtualBox windows to be moved around.
For some reason, the auto capture of Virtual Box seems to conflict with the way how dragging a window from the title bar works
One easy workaround for this is to press the "Host key" (right control by default) to temporarily turn off auto capture mode, so you move the window around normally. (The status of keyboard capture, and the Host key, can be seen in the lower right of VirtualBox)
Just uncheck "Auto Capture Keyboard" was enough for me
Workaround: We can move a VM window when it has not the focus. No need to press the rigth Ctrl. Here is a video recorded: https://video.antopie.org/videos/watch/8c69f9e5-d844-4f80-b30d-01456e2fa6eb