This is really nice! Well-designed and can really help with creating box-shadows in CSS (which, on a side-note, is a real pain!).
I have personally long given up on trying to create a perfect shadow in CSS and just use those available online or in libraries like Tailwind CSS. With that, the "Library" part is a great addition.
One thing is the history handling - with every change writing a new URL to the history - it's really inconvenient for the user who, in most cases, just wants to go back to entirely different page. Consider changing that to not affect the history state.
I have personally long given up on trying to create a perfect shadow in CSS and just use those available online or in libraries like Tailwind CSS. With that, the "Library" part is a great addition.
One thing is the history handling - with every change writing a new URL to the history - it's really inconvenient for the user who, in most cases, just wants to go back to entirely different page. Consider changing that to not affect the history state.