The gif shows it better than I could possibly explain it:

I have not tested every possible value but it looks like the dragging to the end works for frame heights from 100-150 and does not work from 151-199. The values on the left can be edited and accepted with the enter key.

Since the up/down-buttons use the actual entered stepSize and the scrollbar snap does not I am not sure which of the two things is broken.

You can also reach the end by clicking the down-button.

But the up/down-buttons are broken too because of https://bugreports.qt.io/browse/QTBUG-86264 , this example shows this bug as well if you click up/down the items do not align with the top of the frame any more.