Mouse locks when keys are pressed

Solution 1:

Deactivate the «disable touchpad when typing» feature.

In Ubuntu 18.04, at the command line (e.g. in an xterm):

Find id m of touchpad device, called e.g. «Elantech Touchpad»:

xinput --list

Find property id n, probably called «Disable While Typing Enabled»:

xinput --list-props <m>

Deactivate it:

xinput --set-prop <m> <n> 0

Solution 2:

There is an option called "Disable touchpad while typing". It might be mistakenly on. If it is for some reason set to be on, you can disable it by going to Mouse > Touchpad > and uncheck "disable touchpad while typing".

If that doesn't work it might be that your mouse and keyboard do not play nice with one another under ubuntu. Unless these are expensive peripherals, try to reproduce the problem with a different mouse or keyboard or both and see if that changes things.