Interface question on Mac- page up/down, horizontal scrollbar

When a PDF is zoomed out enough such that the horizontal scrollbar disappears, the right/left arrow keys function as page up/down. But when I zoom in so that there is a scrollbar, they move the document sideways. I suppose that is reasonable behaviour. What I find strange though is at which level the horizontal scroll appears/disappears. In my mind, it should only appear when the width of the doc is larger than what is being displayed. But often I have to zoom out quite a bit before it disappears, so there are big grey edges and screen space is not used well. But I still need page up/down. Perhaps this behaviour is a bug?
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