Mir Continues Pushing Ahead With Wayland Client Support

[uid313]

That's the thing, what is this extension? AFAIK, they forked an existing extension, dash-to-dock.

Yes, they forked an existing extension dash-to-dock.
Dash-to-dock have lots of features and does lots of things, which is great for power users.
Ubuntu want a simple dock that provides the Unity experience.
So instead of using the Dash-to-dock extension which has way too many knobs and settings for Ubuntu's design goal they made their own extension, and instead of writing their own extension from scratched they forked Dash-to-dock and stripped it down and tuned it to give the Unity experience.
While also contributing to upstream to Dash-to-dock and pulling down patches from upstream.

Great move by Canonical.
Canonical have done many stupid moves like Mir but this was actually a good move.

[Awesomeness]

Unity is a slimmed down version configured to the vision of Unity.

Yes, they hid most config options. Exactly as I wrote.