When switching the editor to full-screen mode we are currently rendering an arrow button in the left-most part of the header to let people navigate away from the editor and back to the dashboard. This has been debated a few times (notably the discussion in #18387, which proposed adding some options to clarify the action a bit more). It was generally agreed upon that the default state could be a bit confusing but a solution that relies on options for such a prominent part of navigation won't be great.

With site editing on the horizon, there's more of a need for using the full-screen option and it'd be worth working out a better solution to bridge the general admin experience with the editor experience.

This proposal looks at separating the action a bit more from the other editor toolbar buttons (the < blends too easily) by using the WordPress logo as a familiar anchoring point but without compromising the length of the header.

Note this addition would only happen when in full-screen mode (effectively replacing the back arrow). The behaviour of navigating away would remain unchanged.

On focus / hover, we would either show the regular tooltip (like the back arrow does now):

Or we could try something more involved, such as expanding the container:

If it works well, I could imagine this action eventually bringing the main WP sidebar in by pushing the editor aside but that is a story for another time. Thoughts?