How to take a screenshot of a fragment of a web application?

Question

JavaScript

Good afternoon, I am writing a simple photo collage editor. I want to be able to save the image of a fragment of the screen where the user drags his photos. found a link to the html2canvas library, but it seems to work incorrectly. I launch the procedure for taking a screenshot of a fragment with coordinates, but the screenshot is taken from the entire screen.
are there any alternative options to save a fragment of a web application to a file for later publication, or how to get around the pitfalls in html2canvas?