After RESTART every new Safari window no longer reflects its DEFAULT appearance. How fix this? High Sierra MBP
A New window will be the same size as the last one you opened, &amp; will align just below &amp; right of the last window [standard window cascading] - if the previous window was 'new' this session. 
If the is insufficient space to fit the next new window below-right, then it will open aligned under the menu bar, the same distance right &amp; start cascading again from there.

If the last new window was reopened from a previous session, then the sizing will match, but the position will be aligned to the menu bar.

afaik, this is not changeable.