Keyboard navigation changes dependent on page zoom

 * Xreader version 2.2.3
 * Distribution - Mint 19.2 Cinnamon (but running i3 window manager)

Issue
In the newest incarnation of Xreader, keyboard navigation seems to have changed from earlier behaviour and is dependent on the current zoom setting.

I use PDF files like books and so mostly disable continuous browsing. Space / Backspace give me the wanted behaviour: they move the page up / down and stop at page breaks. The next keypress then moves to the next / previous page.

However, if I try the same with Page Up / Page Down (which are located in more accessible positions on my keyboard) sometimes work and sometimes don't. This is highly annoying (for me, this is not meant as an insult!), because it completely breaks my user experience and and I have to start thinking when I rather want to read.

Today I seem to have found the culprit: when a displayed page is smaller than the available window space, Page Up / Page Down work exactly like Space / Backspace. However, when only a partial page is rendered, Page Up / Page Down stop at the page break and will stay there even on multiple key presses.

Steps to reproduce

1. Open a PDF file of your choice (I currently don't think that this behaviour depends on the opened PDF file).

2. Disable the Continuous and Dual ___ viewing modes.

3. Zoom the page so that you can see the whole page and a bit of window margin around it.

4. Move several pages up and down using Page Up / Page Down and Space / Backspace. Both sets of keyboard buttons yield the same results and thus everything works as expected.

5. Zoom into the current page so that only a part of it is seen.

6. Repeat step 4 and note that Page Up / Page Down doesn't move to the next / previous page. This is a different result than the keys Space / Backspace would yield and irritates me.

Expected behaviour
The behaviour on step 6 should be the same as in step 4.

Other information
If you cannot reproduce this, I'll find a free PDF that has the problem on my computer.

Start by reproducing the issue in Xreader with Continuous and Dual viewing disabled, comparing Page Up/Page Down with Space/Backspace at different zoom levels. Trace the keyboard-navigation handling for page boundaries. Done means Page Up and Page Down advance to the next or previous page consistently, including when only part of a page is visible.