'upper-left corner' or 'top-left corner'?

I would interpret "top-left corner" and "upper-left corner" as both meaning exactly the same thing, and would use other words to differentiate between nuances. For example:

• in the top/upper-left corner would imply an area within the square.
• near the top/upper-left corner would imply an area near the corner, which may be inside or outside the square depending on context.
• at the top/upper-left corner would suggest the vertex itself or as close to it as you can get, again, inside or outside the square depending on context.
• etc. ...

Even if there are subtle differences between top and upper in this context, it would be a brave author who would rely on the readers interpreting the subtle differences exactly as expected. If there is doubt as to the clarity, then amplify it to make it clear. Sometimes 6 words or 10 words are better than 3!

I interpret "top-left corner" as the corner vertex, and "upper-left corner" as the area in proximity to the corner vertex. This follows from top's meaning as the extreme element, and "upper*'s as the relative element, of a comparison of heights.

Top:
1.The highest or uppermost point, part, or surface of something.

Upper:
Situated above another part.

However the distinction is subtle, and even my own writings is probably not 100% consistent.