Are you using this or just testing? In practice, I don't want to support this.

I was just curious as to what it would do... I'm happy for this to be closed as "Wontfix".

The only time I ever tried to use this was when the project sidebar wasn't available / usable on mobile and I had no way of creating a workboard.

I think their premise is sound in general, since sites sometimes display full alternate sites (I hate when my wife links me mobile URLs and I'm on a desktop). But we serve the full site with just different CSS, so nothing should be missing.

I'm curious if there's a "one line fix", though I don't know much about the feature.

The feature basically changes the User Agent to desktop Chrome as far as I know, and nothing more. That's why Phabricator doesn't appear any different, because the CSS is already optimized for mobile based on actual screen size (and not redirecting based on user agent).