There has only been one change made over 2 months ago to fix a bug when editing the content.opf file where the position was forgotten when the widget lost focus.
That change is here:
Code:
https://github.com/Sigil-Ebook/Sigil/commit/94a177cac1dc5a5c9562286620b679ff1e28b18f
So please try reverting that change since CSSTab and OPFTab both inherit from TextTab.
If reverting fixes the CSS editing issue, we can move the fix to the OPFTab.cpp and remove it from TextTab to prevent it from impacting CSSTab
Perhaps this is the culprit?
KevinH