[playground] Improve DiffEditor scrollbar + view (#34691)

The previous DiffEditor view of the playground looked broken and not
cohesive. There would be parts of the scrollbar appearing on the left
side for some reason, along with two scrollbars on the right side. This
PR makes the DiffEditor look more cohesive.

Previous:


https://github.com/user-attachments/assets/1aa1c775-5940-43b2-a75a-9b46452fb78b

After:


https://github.com/user-attachments/assets/b5c04998-6a6c-4b52-b3c5-b2fef21729e0