While typing a message, if you switch programs and then switch back,
often the blinking cursor has vanished. This is not the case for the
to/cc/bcc textboxes, only the main body textbox.

It's very frequent but inconsistent. I'm finding it hard to get a clear
handle on exactly what behaviour triggers it to disappear and what
brings it back.

Clicking in the main textbox sometimes restore it and sometimes not.
Clicking into to/cc/bcc and then back again doesn't seem to ever
restore it (appears fine temporarily in to/cc/bcc, but then missing
again once focus is back on the main textbox).

Typing text seems to bring it back. Pressing alt too. Not all keys do
so though, for instance I just tried shift, ctrl and f1, which didn't.