I’ve tried creating 2 way sync with Nextcloud calendar for example - the biggest bummer is recursive tasks.
Nextcloud is a standalone hosted calendar that has both calendar server and user interface for it, whereas Thunderbird is just a user interface (client).
Nevertheless the problems of integrating Nextcloud with Vikunja are pretty much similar to what it would take to integrate Thunderbird.
(Series Creation) As a user, I want to create a recurring task in Vikunja (e.g., “Weekly Team Standup” every Monday at 9 AM), so that it appears as a correctly repeating event series in my Calendar.
(Series Modification - “This and Following Events”) As a user, I want to change all future occurrences of a recurring event (e.g., move the “Weekly Standup” to 10 AM starting next month), so that both Vikunja and the Calendar reflect this “fork” correctly, preserving the history of past events.
(Single Occurrence Modification - “Exception”) As a user, I want to move a single instance of a recurring event (e.g., next week’s standup is on Tuesday instead of Monday), so that only that specific event is changed in both systems, without affecting the rest of the series.
(Single Occurrence Deletion) As a user, I want to delete a single instance of a recurring event (e.g., cancel this week’s standup because of a holiday), so that it is removed from both systems without deleting the entire series.
(Series Deletion) As a user, I want to delete an entire recurring task series from either Vikunja or the Calendar, so that all occurrences are removed from both systems.
Here I posted my braindump that you could talk through AI for more info on what were my thoughts when designing the solution:
Here’s for example a generous fella created the IOS app that syncs IOS Calendar with Vikunja - similar to what it would take to integrate Thunderbird.
The problem is it most likely lacks all those critical functional requirements I listed above.
Some folks seem to like the solution though and I assume because of it’s relative simplicity it’s going to be not that hard to port this open source app logic as Thunderbird extension (?) given that all Vikunja connecting logic is there and calendar syncing, on the other hand, is standardized for the most part.