Just a heads up. I was prompted to install the update today by Software Installer, and went ahead. However, 18.12.3 wouldn't install (and the previous version was uninstalled) because of a missing dependency. After some searching around, I found that it needs an updated version of MLT, and the only way I could get this was to add the MLT repositary, which I had never needed before, and with this in place, it installed OK. I presume that the new version of MLT will be added to the main Kdenlive repositories in due course,

If your distro repository are outdated, we recommend using the appImage version of the software: https://kdenlive.org/en/download/