Check if background-attachment:fixed is supported

background-attachment:fixed is automatically disabled on mobile browsers using webkit eg. Safari. TO get around this, you can use a media query to check if the user’s browser supports background-attachment:fixed.

@supports (-webkit-overflow-scrolling: touch) {

    .wp-block-cover-image.has-parallax,
    .wp-block-cover.has-parallax {
        background-attachment: scroll;
    }
}