Using IDEA as external merge tool in a mercurial graft OSX

I have successfully set up IDEA as an external merge tool in mercurial.

However, when doing a graft in mercurial that involves a conflict, the merge opens up in Intellij IDEA as expected, but on the mercurial end of things, the process assumes its done and goes ahead with a commit and "finishes" the graft (but its not finished!). After merging the file in IDEA, your left with a new change set that needs a separate commit. This is not acceptable, the merge should stay within the graft, mercurial should halt like it does when using mercurial internal merge tool or vimdiff (shrug). The fact that it does work for other tools and when using internal merge tool make me suspicious of IDEA integration (exit code or whatnot)?

Is there someone that could confirm this is not working  because IDEA, or perhaps guide me along the right path here?

Here is my HGRC:

[merge-tools]
idea =
idea.gui = True
idea.args = merge $local $other $base $output
idea.priority = 100
idea.executable = idea

IntelliJ IDEA 2016.2.5
Build #IU-162.2228.15, built on October 14, 2016

Mercurial Distributed SCM (version 3.7.2)

on macOS Sierra 10.12.1