Yes, that's the lens correction tool in action.
I set it back to 0%, and the whole frame comes back.
What a shock. I mean, with the part of the image missing this image would have been dead.
In future, I'll leave a bit of extra space around. I mostly don't crop images and compose in camera, but it looks like I have to change my ways.
What kind of distortion is being corrected there? Isn't the Canon lens, while not a top lens, a pretty good lens? What is being done here?