<p><strong>TL;DR: For some reason the MutationObserver doesn't fire at all on my page. The same console snippet works on other pages, but not when run on my localhost. </strong></p> <hr /> <p>I'm working on a standard react SPA built with webpack and running on localhost. </p> <p>Right now I'm trying to use https://github.com/KingSora/OverlayScrollbars on my page and they mostly work fine, they just don't seem to be able to automatically detect content changes. With some debugging, I noticed that the MutationObserver used internally doesn't fire at all. </p> <p>Then I started investigating further and used this code snippet on my page: </p> <pre class="brush:js;toolbar:false;">let obs = new MutationObserver(changes => console.log('changes:', changes)); obs.observe($0, { attributes: true, attributeOldValue: true, subtree: true, childList: true, characterData: true, }); </pre> <p>However, no matter how I modify the node ($0) or any other child node, the observer never fires. </p><p> <strong>However, it's perfectly fine to use the same code snippet on other pages! </strong></p> <p>Now I have absolutely no idea how this happened, so any help or just ideas would be greatly appreciated! </p>