There isn't a solution for this. The way some of the JavaScript libraries in web pages are able figure out this problem is through the view port.
Example here: https://www.customd.com/articles/13/checking-if-an-element-is-visible-on-screen-using-jquery
See another helpful SO answer here: Get the browser viewport dimensions with JavaScript
Now - Word however uses HTML as a way of formatting - and not as a way of directly displaying things. So even if you could run the same library on the HTML - it wouldn't have the same context.
The best you could do is to get the height of the visible space (which should be the same height as your add-in frame) and attempt to do some mapping. You would have some weird edge cases though, like if the font-size is different, or you have a page-break in the view etc...