This has happened to me on the last 2 versions, but that's all I've used. Entering play mode the top menu flickers, and most of the ui is unresponsive. It takes clicking a lot to get focus. If I get focus on some areas like the project view or switch from game/scene it gains focus and sticks. If I can register a click on the top menu it reverts to the bad behavior once I close the dropdown. If I can register a click on a hierarchy item it gets focus but the rest keeps flipping back and forth and it slows the UI greatly. This doesn't always happen, there is some trigger I can't find and once you hit that it happens consistently when entering play mode until you restart Unity. It triggers fairly often for me, 5-10 minutes or so after I open Unity.