WooCommerce auto-update cart when quantity changed, relates with https://gist.github.com/mikaelz/418a337dace188ba95b476ce65abfe0c

functions.php

/**
 * Auto update cart after quantity change
 *
 * @return  string
 **/
add_action( 'woocommerce_after_cart', 'custom_after_cart' );
function custom_after_cart() {
    echo '<script>
    jQuery(document).ready(function($) {

        var upd_cart_btn = $(".update-cart-button");
        upd_cart_btn.hide();

        $(".cart-form").find(".qty").on("change", function(){
            upd_cart_btn.trigger("click");
        });

    });
    </script>';
}

@gianlucaciralli thank you for the code. However there is a time delay between jQuery("[name='update_cart']").removeAttr('disabled'); and when the actual update-cart button is "enabled" in the DOM, so jQuery("[name='update_cart']").trigger("click"); fails because it happens between those two events. I altered your code as follows:

add_action( 'wp_footer', 'cart_update_qty_script' );
function cart_update_qty_script() {
  if (is_cart()) :
   ?>
    <script>
        jQuery('div.woocommerce').on('click', '.qty', function(){
           jQuery("[name='update_cart']").removeAttr('disabled');
        });
		jQuery('div.woocommerce').on('change', '.qty', function(){
           jQuery("[name='update_cart']").trigger("click"); 
        });
		
   </script>
<?php
endif;
}

While this avoids the problem by adding a little time buffer between the two events, it does not solve the problem and does not assure success. I should also mention that I changed the quantity selector to a dropdown menu and the time delay is variable as it depends on the speed in which the user selects the quantity (the longer the better). I am not good with jQuery at all, is there a way to monitor an attribute status change and only trigger the "click" once an element's attribute has changed?

@ross-bell thanks, your code works perfect and is better than the @gianlucaciralli code

add_action( 'wp_footer', 'cart_update_qty_script' );
function cart_update_qty_script() {
  if (is_cart()) :
   ?>
    <script>
    	jQuery(window).on('load', function(){
    		jQuery("[name='update_cart']").removeAttr('disabled');
    	});
    	jQuery( document.body ).on( 'updated_cart_totals', function(){
		    jQuery("[name='update_cart']").removeAttr('disabled');
		});
		jQuery('div.woocommerce').on('change', '.qty', function(){
           jQuery("[name='update_cart']").trigger("click"); 
        });
   </script>
<?php
endif;
}

This might be interesting, I had similar problems as you folks mentioned and I also had to handle some quantity buttons:
https://stackoverflow.com/questions/47392550/woocommerce-quantity-increment-with-buttons-not-working-after-ajax-refresh-and-a/4740007

Hi guys, anyone have a solution for updating cart totals when is delivery method changed ?

@ghost solution works!

Any ideas how to prevent cart auto update if user tries to add stock for product with insufficient stock?