Contrary to other CalDAV forks, this one is based on the original calendar kolab-roundcube-plugins-mirror/calendar (which means the calendar itself is most up to date) and adds CalDAV capability on top of it. As far as I am aware, it is the most up to date version with the most bugfixes (April 2022). (installation instructions are at the bottom)
Unfortunately, the current situation about CalDAV support in roundcube is quite confusing. There are several plugins (/Forks) around that have CalDAV support but from what I found, all of them are slightly buggy or do not work anymore. All of them are based on https://gitlab.awesome-it.de/kolab/roundcube-plugins, a very old Fork which (as far as I can tell) is based on a version of kolab-roundcube-plugins-mirror/calendar that is over 10 years old. None of these forks incorporate updates from the original calendar, meaning it is only a matter of time until they are not compatible with roundcube anymore. I tried to change as little as possible in the original codebase and only added caldav support as a new driver - which means new updates from the roundcube team should be easy to incorporate.
This is the original calendar that all other forks are based on. It is working very well and is actively maintained but unfortunately, it does not have caldav support
This is the "original fork" of the calendar. A lot of work was put into it and caldav is almost fully implemented. Unfortunately it has a few bugs / problems and most of them were not fixed in any other forks:
A fork of awesome-it to make it work with blind-coder/rcmcardav (a CardDAV plugin) by packing the outdated version of sabre/DAV inside the plugin. But it hasn't been maintained and is still based on a very outdated version of kolab-roundcube-plugins-mirror/calendar.
This is a fork of awesome-it with a few bugfixes to make it work with roundcube 1.3 but its maintainer does not seem to be active anymore. It is the most current fork of the original CalDAV fork. But unfortunately, it is treated as its own project (which means that it doesn't have any updates from the original calendar) and is focused primarily on nextcloud (which I don't really understand since nextcloud is using CalDAV anyway). Also, on top of still having the original bugs included, it is also still based on an ancient sabre/DAV version.
That one confuses me. It is from texxasrulez as well and seems to be the basis of Texxas but was abandoned in favour of texxasrulez/calendar. But it seems to be only a few commits behind texxasrulez/calendar.
All CalDAV forks are based on faster-it which has a very different codebase to the original calendar because of its unfinished "multiple-driver" support. That makes it very difficult to get updates from the original calendar.
So I decided to ditch the "multiple driver" support (which isnt used anywway) and keep most changes in the CalDAV driver itself to stay compatible with the original calendar. I also added a ton of updates:
The original libcalendaring still uses sabre/vobject 3.5.3 In order to be compatible with other plugins (and because version 3.5.3 is ancient), I updated it to version 4.1.5 The problem is, that sabre/vobject makes use of DateTimeImmutable which libcalendaring does not expect. It only needs minor changes to account for that, but unfortunately the roundcube-project does not accept pull requests...
I havent published this as a plugin yet, so you have to instruct composer to install directly from github. Run the following commands in the roundcubemail folder (If you get an error that the "API rate limit" has been exceeded and you need an GitHub OAuth token, just follow the instructions in the console - you will need a GitHub account).
cd /pathTo/roundcubemail
composer config repositories.calendar vcs https://github.com/JodliDev/calendar
composer config repositories.libcalendaring vcs https://github.com/JodliDev/libcalendaring
composer config minimum-stability dev
composer require kolab/calendar
bin/initdb.sh --dir=plugins/calendar/drivers/caldav/SQL