Add loading icon while WooCommerce PayPal Payments process transaction

Solution:

I had similar problem. I wrote the custom jquery plugin and track the changing class on form.woocommerce-checkout

The code jquery plugin

(function(){
    // Your base, I'm in it!
    var originalAddClassMethod = jQuery.fn.addClass;
    
    var originalRemoveClassMethod = jQuery.fn.removeClass;

    jQuery.fn.addClass = function(){
        // Execute the original method.
        var result = originalAddClassMethod.apply( this, arguments );

        // trigger a custom event
        jQuery(this).trigger('cssClassChanged');

        // return the original result
        return result;
    }
    
    jQuery.fn.removeClass = function(){
        // Execute the original method.
        var result = originalRemoveClassMethod.apply( this, arguments );

        // trigger a custom event
        jQuery(this).trigger('cssClassChanged');

        // return the original result
        return result;
    }
})();

The function track the changing form.woocommerce-checkout

jQuery(document).ready(function () {
    jQuery('form.woocommerce-checkout').bind('cssClassChanged', function(){ 

        if (jQuery(this).hasClass("processing")) {
            // stripe is processing
        } else {
            // strpipe isn't processing
        }
    });
}); 

You need to add this code to your theme or plugin in js file