In my experience it's just amateur UI design that causes this. Your display areas shouldn't change size unless the browser changes size. There should be nothing that is "content fitted." That's a historical mistake of early HTML but it's something easily overcome. You really do have to get the HTML+CSS to work like a desktop app before you layout your SPA.

Worse still, applications like microsoft outlook on the web, use the skeleton boxes with comfort animations. What they don't do is pre layout their icons. And different icons will appear in different contexts. I often get the case where I aim for one icon, something will load in, create a new icon, and push my intended target out of the click.

Skeleton loaders are a bad kludge over an initially ignored problem.