ajax.command.customizations.js has an override to the add_css AJAX command that makes it possible to scope styles inside a dialog. This includes functionality to address a limitation of JavaScript's CSSStyleSheet , which has difficulty parsing CSS variables in shorthand declarations when the same style includes a longhand declaration. This logic expects CSS variables to be declared in a previously loaded file, but sometimes (especially with aggregation) , this can happen in the same file.
Checking for a CSS variable value is currently done by checking window.getComputedStyle(document.documentElement
This should additionally check for variables declared earlier within the same file. We're already traversing through a csstree that has this information, so writing that to an object to keep track of varnames/values should hopefully be straightforward to do.
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Follow-up for 📌 Media Library dialog styling Active .
I have this addressed in 📌 Dialog + Media Library refinements to support Gin Active but I'll hold of on closing this issue just in case that one stagnates for some reason