I've released MKVToolNix 6.6.0. I'm trying to get back on a release cycle of roughly six weeks between minor versions (minor being the middle part of the version number). So releases may happen more often than over the summer but contain few changes.
This release fixes a couple of bugs and greatly improves the compatibility to the newly released wxWidgets 3.0.0. A new translation into Portuguese has been added, and you can now re-order chapters in the chapter editor with the usual drag & drop operations.