I just tried to send an email in Gmail, and then it said that I needed to sign in. Since on the screen I was signed in, there is no sign-in link. How do I sign in without losing the email? [I've gone ahead and copied it for now, but I'd like to hope that there's a better solution.]
If you are using a browser that supports multiple tabs then you can open another tab and sign into Gmail in that tabbed window. Then you’ll be implicitly signed in on the tabbed window where you are composing the email, and you’ll be able to save or send it as required.
You can also recreate the condition that you originally discovered by opening two tabbed windows, signed into Gmail. Then start composing an email in one window and sign out of Gmail in the other tabbed window. That will put your composing email tabbed window into the state that you originally discovered (and you can recover it using the above technique).