Oh, this happens only with Firebug console opened. It does not happen with built-in development tools. I'm closing this for now.

Additionally informed contextmenu plugin author about the issue https://github.com/JohnDoh/Roundcube-Plugin-Context-Menu/issues/73. The fix on contextmenu side is needed to make sure similiar errors aren't thrown in other places with the same code path.

Note there's also Save as Task dialog which if contained Attachments tab would also collide. The best solution should fix both and allow extending the Save as Task dialog with Attachment tab later.

Implemented for Files, not for Mail.

For me this should look like:

1. User opens editing session as usual
2. User clicks Save button
3. Error: "You don't have permissions to save. Do you want to save a copy?"
4. "Yes" opens folder selector dialog. "No" does nothing.

An API implemented (not for WebDAV).

The Chwala API for fetching writable folders has been implemented in rCcd9d0eb48a5. So, this ticket is about the Roundcube UI.

Implemented as fileapi_manticore option.

Yes, this is caching. The cache is updated when you log in or visit Settings > Folders.

It is already highlighted by

.calendarmain .fc-state-highlight {
    background-color: rgba(244, 196, 37, 0.1);
}