As noted in #13973 (comment), the URL Popover stylesheet uses a relatively non-standard SCSS method of nesting some classnames. For instance:

.editor-url-popover { 
    &__settings-toggle { 
        ...
    } 
}

... instead of:

.editor-url-popover__settings-toggle { 
    ...
}

This is different from the conventions used elsewhere in Gutenberg, and is a bit more difficult to follow for that reason.

This commit un-nests those styles, and should have no effect on the compiled CSS.