Fix gl_dropdown scrolling to top on assignee click

What does this MR do?

• 👍 Fixed jumping

Before	After

• 👍 Check box still works

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

• Tested in all supported browsers