I run into this situation all the time when working on graphics, 3d models, or converting web wireframes to HTML+CSS: the need to keep a mini snapshot of what I’m mimicking in the same frame as the window that I’m creating it in. Think Picture-In-Picture but for your desktop.
Enter Sticky Previews.
This handy app lets you create an arbitrarily-sized rectangle snapshot from anywhere on your desktop then lets you place and scale that rectangle on another screen or over other work on the same screen. Take a look at a quick 2 frame video showing a Chinese character in my Google Doc then the Sticky Preview in the bottom left of my Blender window. I’ll use this reference to model the Chinese character in 3d.
I purchased Sticky Previews years ago and have used it hundreds of times to speed up my work-flow. If you’re a web developer or designer, I’d definitely recommend it.