This has been going on for awhile for me (at least since 1.7 if not a little earlier). Under the Power tab in the right upper corner, I have not been able to select the last race listed. The scroll bar even looks like it can go just a little farther but it can't. So I have to go open the Diplomacy window in order to communicate with them. I have a resolution of 1920x1080 and the normal sized UI is being used.