CalDav Desktop client with sub-task support?

Does anyone know a good open-source desktop client (Windows/Linux) that supports sub-tasks?

Thunderbird & Lightning is nice, but the task part is not very nice to use, especially due to the missing sub-tasks.

The Pro version of Rainlendar works excellent, but is neither Free nor gratis. Still if that isn’t a consideration for you I recommend to have a look (the Pro version can be downloaded and installed, but adds a sort of watermark that it is unregistered).

But actually a more simple desktop widget or simple app of some sort that displays sub-tasks and has CalDav support would be nice to have as FOSS.

I think KOrganizer supports sub tasks. I don’t know of any others.

Ah, true that one works well under Linux. How stable is the Windows version these days?

Sorry no, have been using Rainlendar and Android apps.

I would be interested as well if there’s such a FOSS solution - it’s the only issue bothering me with Thunderbird. I was even considering to recommend others using the Outlook client with the plugin because of this, can you imagine?