Thanks to both of you. I was afraid of getting this kind of reply: you have confirmed my findings. The User Interface setting does change things and makes the whole display bigger; but the quality of the drawing display goes down horribly. The size change to the texts I identified is relatively small. So the overall effect is negative. Fortunately, I have used QCad quite a lot, so I know my way around - and I can squint when required.
This screenshot is with the User Interface Scale Factor at 1.5. Notice that the cancel X in the tab is 1.5 big, but the text still needs at least half a squint. Now I see it, the quality of the drawing in the screenshot in this post is acceptable. On my 4k screen it is not nearly this good: the quality of the display shows the clunkiness of the drawing very well.
dajo
