Calendar ignores ical DESCRIPTION;ALTREP when updating event description

[cm-schl]

Discussion started at Bugzilla of the Thunderbird Project: 1748002
Preamble: Thunderbird introduced formatting options for event descriptions that are saved via CalDAV as DESCRIPTION;ALTREP. Thunderbird always saves a HTML version (formatted) and a plain text version of the event description like defined in RFC5545.

Steps to reproduce

1. Use Thunderbird Version 91 to create an event with text in the description field, for example the word "test". The event gets saved correctly on the CalDAV server. The description field of the VEVENT contains:
   DESCRIPTION;ALTREP="data:text/html,%3Cbody%3Etest%3C%2Fbody%3E":test
2. Open Nextcloud Calendar and edit the event description text to "1111111111111111test". Now the VEVENT on the server looks like this:
   DESCRIPTION;ALTREP="data:text/html,%3Cbody%3Etest%3C%2Fbody%3E":1111111111111111test
3. Open the event in Thunderbird. The event description only contains "test" because Thunderbird shows the content of ALTREP.

Expected behaviour

After a discussion at bugzilla.mozilla.org (1748002) it seems not correct that Nextcloud updates only the plaintext version of the event description without modifying or deleting the alternative representation under ALTREP. Other CalDAV clients like mobile phone apps seem to delete everything under ALTREP when the plaintext version has been modified by the user.
The problem is that actually the user does not see modifications made via Nextcloud (for example in shared calendars) and therefore overwrites these changes if he tries to update the event description using Thunderbird.

Naturally the best solution would be if Nextcloud Calendar would show the formatted version instead of the plaintext version 😉, but at the moment it seems important that Thunderbird users aren't going to overwrite changes made using Nextcloud.

Actual behaviour

As you can see Nextcloud only updates the plain text description, but the HTML description in ALTREP isn't updated nor deleted.

Calendar app

Calendar app version: 3.0.4

CalDAV-clients used: Thunderbird Lightning, DAVx5, Simple Calendar (Android) for tests

Client configuration

Browser: Firefox 91.4.1

Operating system: Windows 10

Nextcloud Version: 21.0.7

[tcitworld]

Thanks for the bug report.

As a sidenote, we will probably use RFC9073's STYLED-DESCRIPTION for rich text formatting if we ever have this feature (see #3234), not ALTREP.

[benbucksch]

The bug comes from the data being inconsistent now. When the description changes, the parameters of the DESCRIPTION property are no longer valid.

The expected minimal behavior is that NextCloud, when writing the DESCRIPTION property, also deletes all parameters of the DESCRIPTION property, which are now outdated.

So, the fix here should be simple: When setting DESCRIPTION, just delete all parameters of DESCRIPTION property, including the ALTREP parameter. That avoids the bug.

[benbucksch]

• From Thunderbird, before modification:
  DESCRIPTION;ALTREP="data:text/html,%3Cbody%3Etest%20one%3C%2Fbody%3E":test one
• After modification in NextCloud - Actual result:
  DESCRIPTION;ALTREP="data:text/html,%3Cbody%3Etest%20one%3C%2Fbody%3E":test two
• After modification in NextCloud - Expected result:
  DESCRIPTION:test two

Given that the property parameters describe the property (not the whole calendar item, just this property), and the property changed, it's reasonable to assume that the parameters which described the old property are no longer valid.