Lubuntu 18.04 LTS, Dell Latitude 3540 laptop with VirtualBox 5.2.10 installed. Attempting to set up two different Windows VMs both resulted in seemingly random "lock up" of left mouse button clicks within an hour or so. This is not only in the VM, but in the Lubuntu desktop also; total left-click failure. It has only ever happened once with VirtualBox open and no VM open, every other time was with a VM open. When it happens, only the left button is affected; right button and keyboard still work fine.
No obvious method was found to remedy the situation outright, but it was discovered that Ctrl+Alt+F6 to switch out then Ctrl+Alt+F7 to switch back to GUI mode will "fix" it, for a little while anyways. After it happens once it usually happens again within 15 minutes or less. Only closure of VirtualBox, logoff, or reboot will prevent it from happening again.
Any ideas of what to check? The Synaptics touchpad has been challenging to setup with Lubuntu. Compton is also installed to deal with screen tearing, and x-screensaver. Perhaps a combination of these contribute to the issue.