I notice that when I change
\documentclass[a4paper, 12pt, openright]{report}
to
\documentclass[a4paper, 12pt, openright, twoside]{report}
the vertical page alignment shifts to "center", while before it seemed it was "top".

Why is that? Is there anywhere I can declare default vertical alignment to be "top" for the whole document. I prefer not using extra/special packages.
\raggedbottomin the preamble. – Jürgen Jun 21 '17 at 12:35\raggedbottomshowed me the way to this post which answers the "why": https://tex.stackexchange.com/a/10747/31153. I am not sure whether to delete or leave this question since it is a duplicate. – Davor Josipovic Jun 21 '17 at 12:55