Drop the rounded panel corner.
It can easily be hidden in the CSS, but for a more satisfying end it can be completely removed from the codebase.
Edit: The rationale for removal is that it was a dated bit of stylistic flair and to match the updated style of libadwaita it would have to have a greater radius which would both draw more attention to it and increase its overlap on applications not in that style.
Edited by Sam Hewitt