When you change From account and you don't have typed any message yet in the body of your email, it's impossible to write anything anymore. Only the headers fields can be changed.
This is happening when the default email account has a signature and the second one not.
Workaroundd
Add a default signature to all your email accounts or type anything in the message's body before changing the From address
Just added another account to try it out and this doesn't seem to reproduce for me :o Could you share console output (Ctrl-Alt-I inside the composer window) after changing the From account?
What operating system are you using?
Linux, Fedora 31 ...
What version of Mailspring are you using?
Mailspring Libre 1.7.2.1 and 1.7.3 from CI
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Bug?
When you change From account and you don't have typed any message yet in the body of your email, it's impossible to write anything anymore. Only the headers fields can be changed. This is happening when the default email account has a signature and the second one not.
Workaroundd
Add a default signature to all your email accounts or type anything in the message's body before changing the From address