we already checked implicitly
call the requested action to navigate in horizontal mode rather than always use goToPage:
make sure we don't turn unsigned integer negative
correct comment
We cannot overshoot when correcting scrolling on page-up/down, because we only reduce what PDFKit does. Add sanity check. No mneed for intermediate variable.
make sure scrollPageUp/Down and scrollLineUp/Down are not called when PDFView does not implement them
check full bounds origin for changing before navigating for page-up/down in horizontal mode
only bother ssetting ivar when we are using it