Fix subpixel border issue on MR discussions

What does this MR do?

Fixes an issue where MR discussion border is being hidden because Chrome rounds subpixels down.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Links

• Closes #53973 (closed)