Here is me reverting the change after the Sigil -0.8.7 release.
https://github.com/Sigil-Ebook/Sigil...18640a7987f111
It caused some signals to not be properly attached to some slots in MainWindow causing even some primary buttons to stop working seemingly randomly, and lots of hard to detect issues. So again, I strongly recommend using the latest Sigil (especially the forthcoming Sigil-0.9.0) but if that is not possible then use Sigil 0.8.6 over any earlier releases.
Hope this helps,
KevinH