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#1589 page up and down go too far when moving to the next page
Suppose I have a pdf open in "Single Page" view and the page is not fully on the screen, that is, it's zoomed in. When I press page down while at the last part of page, Skim moves to the next page, but not to the top of the next page. It moves to the bottom of the next page.
Apple Preview behaves as expected. If I am at the last half of a pdf page in "Single Page" view and I press page down to move to the next page, Preview changes to the top of the next page.
It is the same for the complementary situation: pressing page up at the top of a page in Skim moves to the top of the previous page instead of the bottom.
Page Up/Down is implemented by Apple;'s PDFKit. So they are responsible for how this behaves. We cannot fix this, even if we wanted to, because we don't have access to all the internals of the scrolling, so we would only add more bugs if we tried. I=f you think this is a bug, you should report this to Apple, not to us, as only they can fix this. BTW, you could use Next/Previous page (Option-Up/Down) to get the behavior you want.
BTW, I have no idea why Preview does behave differently, as they also make use of PDFKit. They must be overriding the scroll methods, they have insider information on how things work that we don't have so they could.
BTW,, PDFKit has a long history of getting page scrolling/navigation wrong, in many different ways, and often differently in different OS versions. In fact, I don't know of a single OS version where they have done this right in all situations. We do correct some scrolling in certain situation where otherwise it would be completely unusable;e (e.g. page navigation in horizontal mode). But for this particular situation it is not possible to fix by us (I have tried a long time ago when PDFKit did it completely wrong, but then our 'fix' turned into a bug in a later OS update because PDFKit decided to handle scrolling completely differently). Moreover, this is not so serious, because there is a working alternative, and it is not completely wrong.
BTW, I have reported this particular bug already to Apple in 2019 for macOS 10.14. And many related scrolling bugs over the years, before and after that. Usually I get no reply (like for this one), sometimes they are fixed but replaced by other bugs. So I wouldn't get my hopes up.
Thank you. Unfortunately, the alternative, previous/next (option up, down) does not work for me. If I am zoomed in such that the top of a page is showing, but not the bottom, the next button does not bring me to the second half of the page. It takes me to the top part of the next page. It's similar with previous, option up, and if I start from the bottom half of the page instead of the top.
In any case, like you said, it isn't a serious problem. I can just press page up or down to move to the appropriate half of the page when needed. I figured just a tweak would be needed to fix this and that since Preview works PDFkit wasn't a fault, but I see that PDFkit is at a fault, and so it's impossible to fix the cause of the problem. A workaround would be much too much trouble for such a little inconvenience. I hope Apple fixes it someday. Thank you for explaining so thoroughly!
I did finally find a way to correct the scrolling that should be safe when Apple changes its mind again. So this should work as expected in the next release.