I love the bookmarks sidebar, but whether or not sections have sub-sections will determine the heading indentation in the sidebar, leading to an unintuitive visual structure.
In the first picture, one of the Heading 3's is aligned with the Heading 2's because it doesn't have any Header 4's underneath it. The expand/collapse triangle pushes the other Heading 3s further to the right so it's off compared to the standalone Heading 3.
My adjusted alignment makes it so that heading levels stay aligned regardless of whether there is expandable/collapsable content underneath it.
It's important to maintain alignment so users can understand the structure of the Article's content just by looking at the bookmarks bar.
Thank you! :)