Two examples: hypothes.is and ferret.ai
There many more of such examples, but they all modify the page content to accomplish this. this is have so many drawbacks that it's not an option for this project. There is now in development Sidebars API by Chrome team (or at least it's marked so in there issues databases) when support for real native sidebars will be available it will be utilized.As for using the Tabs Outliner in the in-page sidebars, maybe exist some extension that can create sidebar for any page (like Tab Resize or Panel Tabs, but they do so in panels), then it will be possible to load the Tabs Outliner interface in such sidebar, provided and managed (and collecting all of the negative feedbacks because of the all accompanying problems) by other extension.
Thanks for the response!
Panel Tabs looks like it'll work well for me! I have a few suggestions for the author of that extension which I'll take up with him/her.
I'm excited for advancements in Sidebars when that comes out!
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