Internet searching turned up nothing at all about this problem, so posting the solution here to save this community some time and headaches.
I have a Lubuntu 14.04 VM in VirtualBox. It had VirtualBox Guest Additions version 5.0.2, where the login screen used the same resolution as after logging in.
I recently upgraded VirtualBox to version 5.1.18. And in my Lubuntu 14.04 VM, when upgrading VirtualBox Guest Additions to the version 5.1.18, the login screen resolution became stuck at like 800x600 or so. Only after logging in does the proper resolution get restored.
Fortunately, the old behavior can be restored. Create a file in /etc/lightdm/lightdm.conf.d named vboxadd-login-screen-fix.conf and fill it with this -
Reboot the VM and the login screen should have the proper resolution again.
I have a Lubuntu 14.04 VM in VirtualBox. It had VirtualBox Guest Additions version 5.0.2, where the login screen used the same resolution as after logging in.
I recently upgraded VirtualBox to version 5.1.18. And in my Lubuntu 14.04 VM, when upgrading VirtualBox Guest Additions to the version 5.1.18, the login screen resolution became stuck at like 800x600 or so. Only after logging in does the proper resolution get restored.
Fortunately, the old behavior can be restored. Create a file in /etc/lightdm/lightdm.conf.d named vboxadd-login-screen-fix.conf and fill it with this -
Code:
[SeatDefaults]
greeter-setup-script=VBoxClient --display