inconsistent page-down [closed]

edit

It appears that the response to 'Page-Down' can seem inconsistent.

Create a document of several lines (which may be blank) followed by a new page. With the cursor anywhere on the first page, but the display at the default position for top-of file, a Page-Down moves the display but not the cursor. If the display is scrolled down a line, then Page-Down moves cursor and display.

Even more bizarrely, this appears to be dependent on the zoom level of the display. At 95% zoom, cursor and display always seem to move.