Posting this in hopes of raising awareness to the issue (and because it was recently reopened). The sidebar API would help open a new world of useful chrome extensions (especially tab management, such as for instance vertical side tabs). If this is something you want, please show your interest by starring the issue.
I think the title to this should've been something more descriptive like "Google willing to accept Chrome Sidebar API patches" as that seems to be the main reason it was reopened.