Homepage Posts block: Grid + borders + image behind don't work together

[laurelfulford]

Describe the bug
If you pair the settings:

• Grid layout
• Border style
• 'Behind' featured image placement

... the border is not really visible. The Featured image fills all the available space, and runs into the border.

This issue has not been reported by anyone-- it's an odd combination of settings, so it's unlikely anyone is using it.

Because of that, it should be considered a low priority; one possible fix could just be to remove the ability to combine those options.

Screenshots

Featured image behind, grid and borders:

Featured image above, grid and borders -- note the visible border: