Last month, I submitted a patch to GNU Emacs with an implementation of line breaking inspired by Knuth-Plass and an option to enable it for paragraph filling.

Last month, I submitted a patch to GNU Emacs with an implementation of line breaking inspired by Knuth-Plass and an option to enable it for paragraph filling. (I'm still iterating on the patch.) My approach to runts was to apply the full penalty to the last line, but only if the number of words was below a threshold (by default, only if it's a single word).