This was just working for me in the past on two different machines (Linux Mint 17.x and VirtualBox 5.x w/KDE). Where if I use a VM in seamless mode it was using the just using the non-taskbar area of the screen for the guest. Now it is using the resolution of the whole screen including behind the host taskbar. So the guest is bigger than the available screen real estate and there is some way to scroll the guest window area by mousing near the host taskbar - I have yet to be able to scroll up so then I lose access to the top of the guest display. I assume I am missing a setting somewhere for this. I also have two monitors if that makes any difference, using both for the VM. One does not have a taskbar on it. Any ideas?
Edit: uploaded a picture to illustrate the issue.
- snapshot1.png (12.05 KiB) Viewed 525 times
Edit2: I found a cheesy workaround for it in that I use a program that hacks the registry to add custom resolutions to the guest OS (Win10) and sets the resolution to 1920x1100 - still pretty far from ideal as it leaves a lot of wasted space but at least it is usable and not covered by the host system task bar. If there is a better solution I would love to hear about it. Still not sure why I never encountered this problem on any of my other systems.
Edit3: I looked at one of the machines where it works (running the same distribution and same (major) version of VirtualBox and a clone of the host system image) and the guest system sees a resolution of 1920x1168. Which is what is missing on the machine where it doesn't work properly in seamless mode. I think there is probably some setting affecting this but not sure what it would be.