Fixed merge on success widget buttons on mobile

What does this MR do?

Fixes 'merge on success' widget buttons on mobile as they were stacked with 100% width before when it is a button group with a small dropdown button.

Are there points in the code the reviewer needs to double check?

Ney

Why was this MR needed?

mobile UX

What are the relevant issue numbers?

Closes #18610 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together