Support auto-fill of logins in Incognito mode
Split off from #952 so it's individually discussable/actionable; @mjog wote:
I have Incognito set as the default http(s) handler, so links opened from other apps like Signal and even web apps like Mastodon open in Incognito by default.
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Support auto-fill of logins in Incognito mode - so if I click on e.g. a gitlab link in my email, I can just log in and deal with the ticket in the Incognito window.
However, note that @mcatanzaro replied:
As currently-implemented, we can't do this because it would be a privacy vulnerability. We currently autofill usernames and passwords immediately when visiting the webpage, which obviously must not be done in incognito mode unless we have some user interaction first. So this would require adjusting the feature to not autofill until the user clicks on the form elements, which would require some design work to figure out how to do without confusing the user (we don't want username and password to surprisingly pop out of nowhere).