If you set some clipping (pure integer coordinates, something like 0,0,20,20) and you have some fractional scaling like via QT_SCALE_FACTOR="1.2" the clipping will cut way one pixel to many for some bad rounding cases.

See comment

https://bugreports.qt.io/browse/QTBUG-66036?focusedCommentId=479441&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-479441

in

https://bugreports.qt.io/browse/QTBUG-66036

One can workaround by just converting the QRect into some QRectF and calling the QRectF overload instead of the QRect one.

But that setClipRect misbehaves in such a way is neither documented nor expected.

Screenshots with illustration of the problem in the linked comment, difference between the shots is just the QRect => QRectF conversion, integral coordinates/widths/heights.