I have a feature request for a toggle that locks scrolling (with touchpad) in the horizontal direction. This is particularly useful when Okular occupies say the left half of the screen and is zoomed in on a narrow fit to only view text and not the whitespace/margins beside it.
I’m currently thinking about horizontal scrolling, but this is probably what Trim Margins / Trim To Selection is good for. (Unfortunately, Trim Margins does not work well yet.)