Xournal, cannot draw with pen/highlighter, leaves only a dot

When annotating a PDF I select the pen tool or the highlighter tool and want to underline or cross out or highlight parts of the text but after click-hold + drag + release I only get a dot at the position of my initial click as opposed to the line I expect to appear. The dot is the right colour and size, though. So, the question is: How do I get the pen tool to draw a line? Versions: Linux Mint 19.3 Tricia (VM on a Win10 host) with Xournal 0.4.8. (I rarely use Xournal and had to find this answer twice now so I leave my fix here.)