[eejdoowad]

What happens when two extensions try to place a widget in the same location? Chaos. Not that the status quo is any better.

Interestingly, Firefox's sidebar API only allows a single sidebar to be shown.

[drdaeman]

I still don't get it. What problem they're trying to avoid?

It's exactly the same if two extensions would currently try to manipulate the page by adding a frame (or, worse, a <div> with non-isolated UI) to the same location.

The only difference is, with out-of-page widgets (e.g. overlay regions or popouts), they won't be visible in the page's DOM.

[eejdoowad]

My understanding is that if they do implement an API, they want it to be better than the status quo, not the same.