Important:
The Guest Additions which come with VirtualBox 5.2.6 and 5.1.32
do not work properly on Linux guests with 3D enabled.
Here are updated versions for 5.2.6 and 5.1.32.
5.2.6 ttp://virtualbox.org/download/testcase/VBoxGuestAdditions_5.2.7-120528.iso
5.1.32 ttp://virtualbox.org/download/testcase/VBoxGuestAdditions_5.1.33-120529.iso