Update: 15 Feb 2021

It seems that the Chrome team has reverted this change, or at least on my end I don't see the white glow anymore. This is on chrome version 88.0.4324.96 though everything else about my setup is the same. I can't say with any certainty whether this was changed in this version or in a previous major version since I made the original question, though, but perhaps updating to 88 will fix it for others, too.

Here's what SO looks like to me now

And my own website

As you can see, no white glow anymore!

Original Post

On Ubuntu 20.04.01 LTS, running Google Chrome 85.0.4183.83, when using the devtools "Device Toolbar" option, I get this horrible white box-shadow around the responsive window. I thought it was my own website and that I somehow set a box-shadow to the html element or something, but testing it on a variety of different websites, the box shadow shows up

It's a lot less obvious when the website doesn't have a dark background, but as you can see from the screenshot of my own website it's very obvious on sites with dark backgrounds, as you can see with Stackoverflow below

It's not immediately apparent on Google's homepage, however if you look hard enough you'll definitely notice a white glow around the responsive window.

What could be causing this? It happens even with no extensions active, with a fresh boot of Chrome/Ubuntu, the only thing I haven't tried yet is reinstalling Chrome, but that seems an excessive measure. There either wasn't this white glow in the past, or I just didn't notice it before now (which I doubt).