ubuntu xfce/lxde 16.04-19.10 virtualbox bidirectional clipboard sharing, window resizing, shared folders not working

A variation of this answer worked. Specifically, this fixes it for lubuntu 16.04:

sudo apt-get update
sudo apt-get install virtualbox-guest-x11 xorg-video-abi-20 xserver-xorg-core

if it asks you about keeping a file or installing the new one, press i to select the new one/package maintainer's version;

sudo VBoxClient --clipboard

this fixes it for lubuntu/xubuntu 18.04 - 20.04 and derived variants:

sudo apt-get update
sudo apt-get install virtualbox-guest-x11

if it asks you about keeping a file or installing the new one, press i to select the new one/package maintainer's version;

sudo VBoxClient --clipboard

Note, as raghavan said, that installing virtualbox-guest-x11 package as shown, also resolves the issue where the shared folders are not visible.

And installing this package (and rebooting the virtual machine) also resolves the guest window not autoresizing when the vm window is resized.

This also resolves the issue where paste after reboot no longer works and sudo VBoxClient --clipboard command needs to be repeated. After installing virtualbox-guest-x11, the --clipboard command does not need to be repeated after each reboot.