Only happens on Safari on iOS 7.
You will notice it the first time you change to landscape mode. That is if you start in portrait, you will see it if you rotate to landscape, and if you start in landscape you will have to rotate to portrait and then rotate to landscape again. The top of the session is slightly obscured by the control bar, and there is a few pixels of blank space at the bottom of the browser.
Toggling the onscreen-keyboard will put the canvas back in the correct place.
Problem does not exist in Safari on iOS 8.1. We should consider closing this bug.
Due to being non-crucial and the fact that it's resolved in Safari 8+ we are going to consider this fixed.
Verified that this issue is still fixed in iOS 8.3 (12F69)