[Waterfall view] Fix the highlight hover board in the row items

### Problem
In the row items, when you hover over an individual item, the border doesn't apply evenly around the entire item.

### Solution
Get the border to fully wrap around a single pixel around the code

Start by locating the Waterfall view and the row-item hover styles, then reproduce the uneven highlight border on an individual item. Adjust the relevant styling so the border wraps evenly around the item, and verify the hover appearance across row items.

Written by the indexing model from the issue text.