[dev-overlay] Consider scrollbar width for drag positioning (#78865)
This PR fixes a small bug with the dragging logic established in #78716
Namely, I didn't take into account the scrollbar width when calculating
`translate` for each corner. So if the page was scrollable the drag end
would be a bit janky:
https://github.com/user-attachments/assets/17e227e9-7af1-4ab8-91b2-b7cc4d363793
After this PR, the snapping is much smoother with no jank:
https://github.com/user-attachments/assets/06021fae-da61-46b3-b224-11f319835fa3