In this specific case, the Emacs maintainers took a mode which already shipped with Emacs (in fact it was the mentioned third-party Prolog mode that was already

In this specific case, the Emacs maintainers took a mode which already shipped with Emacs (in fact it was the mentioned third-party Prolog mode that was already bundled with Emacs with slight modifications) and changed it so that it no longer worked as previously and also not as expected. It is definitely nice that the Emacs maintainers corrected some of these regressions which I explained in this issue with steps to reproduce them. However, the information from 23 Nov 2015 that 2 out of several regressions I had reported in this issue are now believed to be corrected does not solve the other issues I reported in the course of the discussion and which still remain. It is easy to reproduce them with the instructions I posted in case someone is still interested in correcting them. Alternatively, one can simply use the third-party mode which does not have these issues also because I had previously already contributed changes to specifically address some of these issues in that original version of the mode. The key reason for the divergence between these two versions of the same original mode is that Emacs maintainers prefer to use SMIE-based indentation for the mode, and switching the mode to SMIE in the Emacs trunk introduced the reported regressions. SMIE is an indentation and navigation engine that the Emacs maintainers provide. My impression from the discussion in the cited issue is that SMIE is lacking some of the features that are needed to implement the desired and previously supported indentation and navigation for this particular mode. There may be a way to improve SMIE to support these use cases, it may be quite hard though: I expect that if it were easy, these changes would have been made by the Emacs maintainers to solve the remaining issues too in the last few years.