This was discussed elsewhere in the comments:

> This can be solved with CSS. Extend the background blur all the way through the element and then use CSS masks to cut out the actual shape you want.

> With this, you can remove the border (or inset box shadow), and the edge of the glass will look much, much more real

I tried this and it works! One unfortunate impact is a loss in simplicity. In the example on the page you can apply the non-JavaScript version to pretty much any element and get a nice glass effect with `border-radius` and such still functioning as expected.

Using `clip-path` I'm able to consider background pixels more correctly but it looks like I'd need an extra div and/or some sizing tricks to get everything working exactly as expected.

I'll keep noodling on this and may add an update to the page if a simple solution comes to mind.