Bookmarking an on-screen position in macOS?
A possible alternative to using a Finder window as a bookmark is using the built-in Stickies app (if you are not using the apps as a fullscreen space).

Simply open the Stickies app and make a new note via File > New Note. While the sticky window is selected, you can make it float on top of other windows at Window > Float on Top. This will make the sticky note remain on top of the other window, regardless of app focus. You can resize the sticky to your liking and you can even collapse it to make it much smaller.

It's not an elegant solution and it falls apart if the sticky or the window are moved, but it is better than using Finder as a bookmark.

Here is an example of what it looks like floating on a list of numbers on an active Chrome window: