Create a program that zooms the screen underneath the mouse pointer using a set radius and set zoom level (%), that follows the mouse pointer as long as a particular keyboard key or key combination is depressed (and goes away upon release).
Ideally the cursor would not be part of the magnification.
This is to facilitate onscreen teaching, such as https://youtu.be/b7m8GQn33u0?t=1259
(apparently there is something like it for mac). Windows magnifier is square rather than round and is hard to start and stop.