No warning message when last Thunderbird window is closed on Mac

[jikamens]

On Windows and Linux, when you close your last open Thunderbird window, the application exits, and if you have any messages scheduled with Send Later, a warning is displayed reminding you that scheduled messages can't be sent when Thunderbird isn't running.

On Mac OS, however, it is possible to close the last open Thunderbird window without the application quitting. When you do this, then Send Later doesn't display any warning, but the Send Later background code that delivers scheduled messages is no longer running, since it only runs when at least one main Thunderbird window is open.

We should figure out a way to get Send Later to display the same warning when the last main Thunderbird window is closed on a Mac, that it displays on other platforms.