Use case or problem
The Structure panel indents a header when it has children.
This makes eye-navigation of same-level headers more difficult (because the ones without children stick out to the left).
Example:
“Title 1” is at the same H1 level as “Title 2”, but the Structure panel makes them seem like they are on 2 different structure levels (only because “Title 1” has sub-children):
Proposed solution
Style the Structure panel so that same-level headers are all indented the same, and put the arrow on the left of the header without modifying its indentation.
Example (photoshopped):