you can already adjust the width of the structure and element panel, when you are in the builder - just by dragging and dropping the panel border. Or am I misunderstanding something?
Yes, I am aware of this. what I mentioned helps us to set the default width of each panel in advance.
Regarding the second part of this request, it disables changing the width of the panels (as you showed) to prevent the width of the panels from changing (unintentionally).
I see. Let’s wait for more feedback on this, if it’s something that more users would need. Personally, I don’t think it’s needed, but as every feature, it might have its own use cases.
As a temporary solution, you can enqueue this CSS to the builder itself: