A filter has been added to select topics that have been solved or not. The ability to mark a topic as solved has been around for a while. However, the latest update appears to have added this filter.
Marking a topic as the solution is buried under the ellipsis at the bottom of a topic. Click it then select solution.
This is a quick way to let others know the issue has been fixed. BTW, you can also say thank you just by clicking the heart.