I have had this problem on my AVH-X8500 and my previous stereo AVH-P3300 while using my iPhone 4S

 

Periodically, randomly, the output display (Usually Waze but I've had it happen with other apps to) will reduce in size so it's not taking up the whole screen; there's about 5mm from each side and rounded corners that's black.

 

Anyone else seen this? Does anyone know why it's happening, or better yet, how to fix it?