Account creation password box requires manually typed entry (won't accept lastpass auto entry)

When creating an account, I have lastpass generate a password and paste it into the box. The box is looking for keystrokes and the button to proceed doesn't light up until I copy/paste the password back out of lastpass and into the box.

Start by reproducing the account-creation flow with a LastPass-generated password, comparing direct entry with password-manager insertion. Trace the password box validation and the proceed button state; done means generated passwords are accepted and the button enables without copying the value back manually.

Written by the indexing model from the issue text.