the touchpad on your laptop is around the size as the screen on your smartphone. They use hand/wrist-sized motions. This is fast and efficient.
Scaling a touchscreen up to laptop or desktop size is a completely different set of motions. Your whole arm gets involved and your fingers have to travel much further to achieve the same motion.
I think the mapped indirect manipulation of a trackpad makes a lot more sense from a human factors perspective.
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I do think the ergonometrics of using touch on a large(ish) vertical display - "Gorilla Arm"[1] - is a deal breaker for extended interaction.
There would be some subtle feedback on the main screen indicating where the loupe is currently focussed I'd guess.
Why do you assume the motions are different? Apple has already proven this with the Magic Trackpad for desktops.
The Magic Trackpad is indirect scaled motion like the inbuilt trackpad on a laptop. It's size is still within the range of easy and quick wrist motion unlike touching the desktop screen itself.
There's even push back against ever increasing phone screen sizes. They've gone beyond a finger's range of motion.