Resolving 3rd party proto file on IntelliJ IDEA

I use Protobuf Support Plugin for syntax highlighting. However, it can't resolve 3rd party proto file like this:

In my case, the file which is not resolved is located in proto/third_party directory.

Solution

1. Open Project Structure (File -> Project Structure or Cmd + ; )
2. Choose the directory as Resource root

Finally, unresolved error was gone 🎉🎉🎉