The practical problem with this idea is that you would essentially need to remove the ability of JavaScript to track all human input (mouse movement, clicks, etc.) since that could already be used to figure out what they’re selecting. Sure, browsers could try to hide this info with clever rules like not reporting mouse movements while the mouse button is being held and text is being selected, but it would be pretty hard to get right (and would probably break a lot of sites that aren’t doing anything nefarious, like any rich text editor).