Really incredibly happy about this! It has been worth the wait for sure. Great work Fastmail!
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However... I did have a weird hiccup with my calendar recently. When they made the changeover this last month, one of my calendars would show up in the calendar list, but events in the calendar would not display no matter what I did. Deleted that calendar and installed a recent .ics backup of it I had made through fastmail, and it did not work either. I had a version if this calendar in the MacOS calendar application, so I did a fresh export from that and it worked when uploaded. It was probably the oldest of my calendars, so I'm wondering if there was just some old code there that didn't play well with the new system somehow. It was a quick fix anyway.