Hello everybody,
quick update on were we are with this: After some toughs we realised the forth panel might indeed be a good solution for both backlinks and TOC but comes with an usability problem when the editor width is too small to fit both text and and forth column (@krssno this pretty much the layout problem you expressed, but with some differences). We are exploring the idea of threshold editor width and displaying the popover when width < threshold and the 4th column when the width > threshold.
This should also works on iPad’s views: Full size → 4th column, medium, small, slide over (split view) popover.
At the moment we are in the process of releasing a public Bear 2 macOS beta, the step after is releasing iOS and then work on the above. Along the way we’ll try to send mockups ideas of the 4th column here on the forum to get feedback.