[DevTools] Adjust the rects size by one pixel smaller (#34876)

This ensures that the outline of a previous rectangle lines up on the
same pixel as the next rectangle so that they appear consecutive.

<img width="244" height="51" alt="Screenshot 2025-10-16 at 11 35 32 AM"
src="https://github.com/user-attachments/assets/75ffde6f-8cc6-49c1-8855-3953569546b4"
/>

I don't love this implementation. There's probably a smarter way. Was
trying to avoid adding another element.