UPDATE from Rovio

8/20: Somethings amiss with this latest update. The menu does not fit into the allotted screen, and clicking on the "screen size" button does nothing but stop the app.

8/23 email from ROVIO: "You could try reinstalling the game. That will not erase your score or progress, but if you want to be safe, you can back them up by copying the settings.lua and highscores.lua files from: ~/Library/Application Support/com.Rovio.Mac.AngryBirdsrio/ Uninstall the game and then reinstall from Mac App Stores Purchased apps page."

Havent tried this yet; a little hesitant. Worried I will be charged a 2nd time for the game.

9/30: Did exactly what Rovio said; game performed as before, so wrote them again.
10/4: Heard from Rovio. They said play the game FULL SCREEN. Button for such in lower right sometimes doesnt work; however, when I clicked "full screen" on the Menu Bar, that worked. NOTE: When game opens, it says: "Minimum 1024x768 resolution needed for optimal gameplay experience."

If you try all these things, perhaps the game will work for you, too.

AmméLeep about Angry Birds Rio