Page Up & Page Down problems

Sigil (an ePub editor) has a problem with its Book view, which, unlike code view, gets neglected. The Page Up & Down keys act like the Home key. The Home Key and End Key work as expected. Clicking in the scroll bar and moving the scroll bar thumb work as expected.

Telling Keystroke Maestro to simulate Page Up and Down keystrokes is of no value, because the simulated keystrokes do no better than pressing the keys.

I mapped Page Up and Page Down keys to single mouse clicks a few pixels from the top and bottom of the scroll bar. Those clicks generate one page shifts. Sometimes that works. Often it fails. Sometimes it works and then fails in the same session.I have tried this method in QuicKeys, and it has the identical problem.

Why would a simulated mouse click sometimes work and sometimes not? Manual mouse clicks always work. Is there a more reliable method to interact with a scroll bar?