The Element's sidebar can't be resized anymore. It seems the resize div is floating in the middle of the elements tree somewhere.
Maybe this was caused by the sidebar migration to Panel?
Created attachment 41784 [details]
Hehe, the patch itself looks good to me. The only thing that caught my eye was something that wasn't changed, but showed up in the context:
> + updateMainViewWidth: function(width)
> // Should be implemented by ancestors.
"ancestors" means ↑ (up). Really this should be the children (sub "classes") that implement it.
Comment on attachment 41784 [details]
Nice. I have nearly identical patch locally.
> "ancestors" means ↑ (up). Really this should be the children (sub "classes")
Oops. Should be subclasses.
Committed r50033: <http://trac.webkit.org/changeset/50033>