There are no Guest Additions for Mac OS guests in Virtualbox. This means no seamless full screen mode for Mac OS Virtualbox guests. This is my workaround.

The first option is to change the resolution according to the available presets with the VBoxManage command.

Be sure to shutdown the VM first! 
VBoxManage setextradata '[VM_NAME]' VBoxInternal2/EfiGopMode [N]
  • Replace [VM_NAME] with the name of the Mac OS virtual machine
  • Replace [N] with the corresponding number for the screen resolution
EfiGopMode Resolution
0 640x480
1 800x600
2 1024x768
3 1280x1024
4 1440x900
5 1920x1200

The command should look something like this:

VBoxManage setextradata "MacOS" VBoxInternal2/EfiGopMode 4

But there’s more!

I found out you can also change the resolution to 1680x1060. Which is a lot closer to 1920x1080. This is the command:

VBoxManage setextradata "[VM_NAME]" VBoxInternal2/EfiGraphicsResolution "1680x1050"

To check if the setting got accepted:

VBoxManage getextradata "[VM_NAME]" VBoxInternal2/EfiGraphicsResolution

Remove the ToolBar at the top of the screen

Open the settings of the Mac OS virtual machine and disable the Mini ToolBar for ‘Show at Top of Screen’ in the ‘User Interface’ section. Now it will only show up when pressing the Host key. I am using [right-ctrl Home] as the Host key.

Run it in full screen

When running the Mac OS virtual machine, make the window full screen e.g. by pressing F11. This results in the screenshot below.

To access the virtual machine controls, press the Host Key. This makes the ToolBar appear. When using the i3 tiling window manager, I had to first press the Host Key to make the window go full screen with [windows-key f].