This will be easiest to duplicate on a really large map like insane with LOOOOOTS of other races so you have time, but it's existed since beta I think 
- start a single player insane map with a crapton of races (I use like 20+ these days)
- order of this step seems irrelevant: meet one or more races & research universal translator
- click end turn
- during the phase where the ai is doing it's thing, go into diplomacy screen -and- speak to someone
- before all the ai finishes doing its thing, cancel back to the map
- screen will go black with the custom gc3 cursor as expected when the turn finishes
- sigh & end task because you forgot to wait... again
- there was one time I got a message about waiting for other player (with an ai in single player game) & just left it sitting while I went to make coffee. it did not happen after I got back that time