Why Screen Corners Are Effectively Infinite-Size Targets

The correct answer is "Screen edges/corners stop the cursor, acting as infinite-size targets."

For a pointer-based interface, screen edges and corners are effectively infinite-size targets: the cursor can be thrown at the corner without stopping precisely, because the OS boundary itself stops it. This is exactly the "Fitts's Law corner trick" — you don't need to aim carefully at a small drawn icon when the edge of the screen does the stopping for you, which is why macOS places the menu bar and traffic-light controls flush against the top edge and corner rather than a few pixels inset.

The distractors don't match the mechanism: nothing exempts corner elements from Fitts's Law — the trick is an application of the law (effectively zero acquisition error at the boundary), not an exception to it; there's no OS behavior that enlarges a control drawn near a corner — the boundary itself is what does the work; and the trick isn't touchscreen-specific — it's a cursor-based interaction pattern, since touchscreens don't have the same boundary-stop behavior for a finger.