Bookmarking an on-screen position in macOS? When looking at something in macOS, how can I put some kind of horizontal line so that I don't forget my position on the screen?

Let's say I'm reading a list shown on the screen,
and I've mentally processing the bottom quarter of the list, so I want to put a marker there so that when I come back from lunch I know where I need to resume from.

My ad hoc solution so far has been to position a Finder window at the point where my region starts. But I end up using Finder for other tasks and can easily loose my "bookmark".
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: