How can I get the user's selection's center anchor point position (x, y) relative to the document/window?
I only know how to get selected nodes using window.getSelection(). But not the position:
You can use getBoundingClientRect() for this purpose. To get the center point, add the half of width to the horizontal distance from the left edge of the screen.
let coords = window.getSelection().getRangeAt(0).getBoundingClientRect()
let center = coords.x + ( coords.width / 2 )
To get the accurate vertical position from the top of the document.
coords.y + window.scrollY