reTerminal DM touchscreen mouse calibration off

I purchased a ReTerminal DM. I’ve installed a virtual keyboard on it (matchbox-keyboard). This has broken the touch screen mouse - it no longer goes to where I click, but instead moves to locations well away from the touched point. Also, when dragged, it selects a rectangle on the screen.
What should the contents of config.txt look like? I can only find examples with dtoverlay=reTerminal, not dtoverlay=reTerminal-plus.

Is there a calibration procedure?