Letter box goes beyond border

[Bill Ob]

As I'm reading an article, all of a sudden the print blows oversized so I can't read the text. It also draws a border on the edge of the screen outlining where the shape of the letter box should be. Does anyone else have this problem? Any suggestions as to how to fix this? I have to close the app and restart it to get back to the normal size of the print. I'm using 5.01 Lollipop.