I don't think this type of action is supported. My first reaction would be that I don't see how to have an appointment open to drop onto whilst dragging a message.

I'm not sure I'm following your last sentence. Open an appointment window in calendar, move it so that you can see the email listing, grab the email and drag it into the appointment window and drop it. In outlook, this is commonplace, and outlook is a crap email/calendaring client. Perhaps if I could find a way to have calendar always display the attachments panel, that's the target for the drop, but I can't seem to find a way to tell it to always keep an attachments pane open. I can even drag a thunderbird email into an outlook calendar appointment window and drop it anywhere, so the problem isn't thunderbird, it's lightning.

Let's put it another way; in all the time I've been using and helping others with Thunderbird, I have never heard of anyone doing it this way, with drag and drop. That's not to say it isn't supported, but I'd hope I'd have heard about it before now.

Thunderbird has been moving towards increased use of tabs for some time, and I for one have spoken up in objection to this trend on many occasions. The Calendar is already in its own tab, and there seems to be much enthusiasm for the Write window and the Address Book to become tab-bound too. Drag and drop, and the ability to browse one whilst looking at another will all become more difficult.

The mutual exclusivity of tabs doesn't encourage the use of a drag-and-drop metaphor when it is made near impossible to see both the source and the destination of a drag and drop operation at the same time. For this reason, I suspect that the design hasn't anticipated users wanting to drag messages onto appointments.

I see where you're going. I'm drawn to Thunderbird because it puts open emails in separate tabs, which makes finding a particular open message easier than searching the task bar for an open outlook message. I do see your point that tab happy programmers, probably too young to remember how cool we thought drag and drop was when it first came out, don't think much of it. As with a lot of software, each programmer has a very limited view of how it their program should be used vs how it is actually used in the wild.

People work in different ways. When I schedule an appointment with a client, this is often preceded by a series of emails related to whatever the client wants to discuss. I drag and drop those messages into the appointment window so that on the date of the appointment, I have a nice neat summary of the issues right there in the appointment window. I could use workarounds (save email and then attach to the appointment), but that's like a trip back to Windows 3.0 and it's, if I'm not mistaken, 30+ years later. I guess I'll continue to use Tbird for email and outlook for calendar, not my preferred solution, I'd like to flip MS the bird and cut all ties to outlook, and was hoping Tbird was the solution. Not yet, and perhaps not ever, based on your thoughts.