Clicking on "bookmarks" icon after clicking on other icons can generate a second sub-window filled with bookmarks. After that, it can get snarled up and the sub-windows will disappear and nothing is visiable.
Using sidebar in a window causes problems with "MRU" tools - "Most Recently Used" tab order switching. I'm guessing Sidebar+ steals focus somehow and causes the tab switching to have nothing to switch between (Using QuicKey, with AHK helper per their github website).
On Win10:
Congrats on new extension. And Good Luck to you!