Here are two examples from a computer book that are used right on the same page (to be more specific, the examples come from this book: Access 2013: The Missing Manual by Matthew MacDonald):
The logo is a tiny picture that usually sits in the top-left corner of a report, next to the title.
A little later on that page it says:
The logo and title typically sit at the top of your report.
I was taught that you should always say at the top-left corner of something, but in the first of the two examples the author, as you can see, says in the top-left corner of. Do you think his English there is totally fine? Maybe, his choice of words has something to do with the fact that there is some sort of spatial implication that the thing he's talking about is contained within boundaries rather than at a certain point as is the case with the at version (the corner of something can be thought of as a point and for points we usually use at)?