To turn off the pop-up on screen keyboard during Drawpile, go to Services, and stop the “Touch keyboard and handwriting panel service”. Once done with drawpile remember to start the service again, as Windows 10 requires the service for it’s internal apps for you to type anything at all.
Press Windows-key and search for service
Look for Touch Keyboard and Handwriting Panel Service and right click to get to Properties
Under Startup Type choose Disabled