Problem
When editing posts, the "Save Edit" / "Cancel" buttons

are slightly off their intended positions like depicted in this screenshot:

Observations
This happens under certain conditions, mostly observed when editing posts. It hasn't happened when adding new posts or when replying.
It looks like this does not happen with short posts on the first position i.e. where the distance from the top of the page is short. Saying that, it does happen with first posts which are vertically larger and also usually happens with subsequent posts on a page, as they usually do have a reasonable distance from the top.