I am struggling with strange behaviour of “inset box-shadow”.
There is some 1px-wide line of background-color on the edge of div, and it is not affected by shadow.
Please take a look at the code to understand what i mean: https://jsfiddle.net/g5vgant8/
(Try to resize viewport if all seems ok, you should see that ugly lines)
You can stop fighting with it now. There’s technically no way to get rid of it.
What you’re seeing is a rounding error as the <div>s resize. The browser has to resize the div and it’s shadow.
You can hide it with a border though. border: 1px solid #440000; removed it for me. If you are using the same colour for outer and inner shadows, the border should remain hidden: