js: introduce ComboScrollY global function (#45040)

Launch callbacks when page scroll below or above a limit

combo/public/static/js/combo.public.js

@@ -76,6 +76,44 @@ function combo_modify_query_string(name, value) {
     return search;
 }
 
+/*  Launch callbacks when scroll move above and below a limit
+ *     new ComboScrollY({
+ *       limit: int,
+ *       below: callback function,
+ *       above: callback function
+ *     })
+ */
+function ComboScrollY(options) {
+  this.defaults = {
+    limit: 0
+  };
+  this.options = options;
+  this.init();
+};
+ComboScrollY.prototype.init = function(){
+  this.options = $.extend({}, this.defaults, this.options);
+  this.callbacks();
+  window.addEventListener('scroll', this.callbacks.bind(this));
+};
+ComboScrollY.prototype.position = function(){
+    return (window.pageYOffset <= this.options.limit) ? "above" : "below";
+};
+ComboScrollY.prototype.update = function(){
+    return (this.position() === this.last_callback_position) ? false : true;
+};
+ComboScrollY.prototype.callbacks = function() {
+  if (this.update()) {
+    if (this.position() === "below") {
+      this.options.below();
+      this.last_callback_position = "below";
+    } else {
+      this.options.above();
+      this.last_callback_position = "above";
+    }
+  }
+};
+
+
 $(function() {
   $('[data-ajax-cell-refresh]').each(function(idx, elem) {
     var $elem = $(elem);
@@ -247,10 +285,11 @@ $(function() {
   $(document).on('combo:cell-loaded', function(ev, cell) { prepare_foldable(cell); });
 
   /* add a scrolled class to body once the user scrolled the page a bit */
-  $(window).on('scroll', function() {
-    if ($(window).scrollTop() == 0) {
+  var body_is_scrolled = new ComboScrollY({
+    above: function(){
       $('body').removeClass('scrolled');
-    } else {
+    },
+    below: function(){
       $('body').addClass('scrolled');
     }
   });