Thanks for filing the issue!

@Reporter: Could you please share a sample test file/URL  which helps us to triage this further in a better way.

Hi,

Thank you for looking into this! I'm not sure if it's just me abusing CSS variables or if it's something else. The title for this issue is not entirely accurate; devtools is still responding, although it becomes really slow. I've put up a preliminary version of the PWA i'm working on: https://ca11.io/ 

This hopefully reproduces it. The app has all variables in the :root scope, so they can serve as a replacement to a multitude of media queries I had sprinkled around.
Just start a session (any username/password) and inspect elements, reload the page, resize devtools. At some point, my laptop is about to take off and the whole process is slowing down. With just the console opened, the browser remains very responsive. Let me know if you need more information. 

It looks like you have about 100 rules each with 15 or so properties. The styles sidebar isn't prepared to handle more than a few hundred properties without lagging. In theory we can do some fanciness in the styles sidebar to hide some rules when there are too many, or hide rules when they scroll out of view. But that isn't a trivial change.

Looking at https://ca11.io/css/app.css , it appears that the duplicated rules are in your source. Each media query appears 37 times. Something wonky appears to be going in your build step.

Oops, that explains it indeed. Sorry about that. I totally failed to make the link with the css output. Mea Culpa :)

Before, the variables sass file only included sass variables. Including this variables file in other Sass files didn't impact the css output. This changed, when I started defining css variables in the same file.