In toolbar CSS, icons get their images by using url() to retrieve an image background: url(../some-image-file.svgetc). In instances where different images were used for pseudo selections such as :active, :hover, those images would not load until the new state was triggered, resulting in the icons briefly vanishing while the variant image was loaded.
This icon-vanishing issue was addressed by including toolbar icon css files in Drupal core's PostCSS build, which uses PostCSS-url, to inline images requested via url(), which eliminates the wait time by providing the image within the already-loaded CSS.
The default Drupal PostCSS build includes the pxtorem process, and that process is bypassed for icons CSS files as the specific px values are needed for icon positioning.
