I don't think we should support multiple sidebars in a single tab - it's
wasteful with web contents, and this "active/inactive" stuff complicates the
code a lot. Why not just have it behave similar to popups (though on a per-tab
basis) - you can have one sidebar open at a time, and if you open another, it
closes the first (completely)?
From https://codereview.chromium.org/1152613003/diff/40001/chrome/browser/extensions/sidebar_manager.cc?context=10&column_width=80&tab_spaces=8