Thunderbird "Download the rest of the message" is not working

Another feature of Thunderbird that I suspect has just been broken in version 60.x is the "Fetch headers only" option.

For as long as I can remember, this has been my setting of choice, but now its broke!

Has Thunderbird lost its Calendar?

A recent Thunderbird update on Debian/Ubuntu systems has resulted in the loss of the Calendar.

As it is the only calendar that I use, I'd be lost without it.

So what's the fix?