Windows 10 Home guest with Guest Additions installed. When guest additions were installed I am able to resize screen size by dragging the window. When Windows 10 screen saver kicks in, the screen size resets to 800x600 and dragging the window does not resize the Windows 10 desktop. Only way to fix it is to reboot the guest.
I know it's kinda silly to have screen saver on a guest and I have since disabled it. I just wanted to inform the community of this issue.
FYI - I don't think the host matters. I have Windows 10 Home running under both Windows 7 host and Mint Linux host and both experience this issue...