Go to
https://en.wikipedia.org/wiki/Evolution_of_insects?veaction=edit
and scroll down by repeatedly hitting the PageDown key. You will notice that it is not possible to scroll past the Reference section to reach the External Links section. To reach the External Links section one needs to use the scroll bar or the CursorDown key.
The reason seems to be that the PageDown action attempts to place the cursor at the first line of the screen, and since the entire Reference section is larger than one screen, this attempt causes the screen to scroll back up to the beginning of the Reference section.