Pressing space in a text editor tends to add a space. Pressing pgdn doesn't.
C-v and M-v in Emacs and friends and CTRL+b and CTRL+d in vi and clones.
EDIT: and if you were fine with reaching for PgDn then SHIFT+up, SHIFT+down in vim is probably a viable option as well.
Great. Now not only do I have to use two keys in everything to do it, it's a different pair of keys in everything! Man, what I've been missing all these years having separate keys for this.
(Yes, I know fn+arrow does them the same in more or less everything as well)