Thunderbird - downloading truncated messages

Please allowThunderbird to download truncated messages while 
simultaneously reviewing other messages.
This functionality was built in to Eudora, for instance, long before the 
turn of the century.

It is very frustrating to click "download the rest of the message" and 
have to sit there the entire time waiting for it to download.  If you 
click on a different message the truncated message download never 
completes.

Thanks,
Dave
re: Thunderbird 45.7.1