add Event.scrollend polyfill (#30)

This is roughly based on : https://github.com/argyleink/scrollyfills
So also attributed to and licensed as this original work.

The original however was slightly broken and didn't implement the `once`
option.
It also lacked tests.

---------

Co-authored-by: Simon Menke <simon.menke@gmail.com>

polyfills/Event/scrollend/config.toml

@@ -0,0 +1,40 @@
+#### Aliases
+aliases = []
+
+#### Dependencies
+dependencies = [
+	"Event",
+	"Set",
+	"WeakMap",
+	"smoothscroll"
+]
+
+#### Specification Link
+spec = "https://html.spec.whatwg.org/multipage/webappapis.html#handler-onscrollend"
+
+#### Documentation Link
+docs = "https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollend_event"
+
+#### License
+license = "ISC"
+
+# The polyfill code was ported to ES3 for compatibility with older browsers.
+repo = "https://github.com/argyleink/scrollyfills"
+
+#### Browser compatibility
+[browsers]
+android = "*"
+bb = "*"
+chrome = "<114"
+edge = "*"
+edge_mob = "*"
+firefox = "<109"
+firefox_mob = "<109"
+ie = "*"
+ie_mob = "*"
+opera = "<100"
+op_mob = "<76"
+op_mini = "*"
+safari = "*"
+ios_saf = "*"
+samsung_mob = "<23.0"

polyfills/Event/scrollend/detect.js

@@ -0,0 +1 @@
+'onscrollend' in self && (self.onscrollend == null || typeof self.onscrollend === 'function')

polyfills/Event/scrollend/polyfill.js

@@ -0,0 +1,132 @@
+(function (global) {
+	var scrollEndEvent = new global.Event('scrollend');
+	var pointers = new global.Set();
+
+	// Track if any pointer is active
+	global.document.addEventListener('touchstart', function (e) {
+		for (var i = 0; i < e.changedTouches.length; i++) {
+			pointers.add(e.changedTouches[i].identifier);
+		}
+	}, { passive: true });
+
+	global.document.addEventListener('touchend', function (e) {
+		for (var i = 0; i < e.changedTouches.length; i++) {
+			pointers.delete(e.changedTouches[i].identifier);
+		}
+	}, { passive: true });
+
+	// Map of scroll-observed elements.
+	var observed = new global.WeakMap();
+
+	function onAddListener(originalFn, type) {
+		var scrollPort = this;
+
+		var data = observed.get(scrollPort);
+		if (data !== undefined) {
+			data.listeners[type]++;
+			return;
+		}
+
+		var timeout = 0;
+
+		data = {
+			scrollListener: function scrollListener(evt) { // eslint-disable-line no-unused-vars
+				clearTimeout(timeout);
+
+				timeout = setTimeout(function () {
+					if (pointers.size) {
+						// if pointer(s) are down, wait longer
+						setTimeout(data.scrollListener, 100);
+						return;
+					}
+
+					// dispatch
+					if (scrollPort) {
+						scrollPort.dispatchEvent(scrollEndEvent);
+					}
+				}, 100);
+
+			},
+			listeners: {
+				scroll: 0,
+				scrollend: 0
+			}
+		};
+
+		originalFn.apply(scrollPort, ['scroll', data.scrollListener]);
+		observed.set(scrollPort, data);
+	}
+
+	function onRemoveListener(originalFn, type) {
+		var scrollPort = this;
+		var data = observed.get(scrollPort);
+
+		if (data === undefined) {
+			return;
+		}
+
+		data.listeners[type] = Math.max(0, data.listeners[type] - 1);
+		// If there are still listeners, nothing more to do.
+		if ((data.listeners.scroll + data.listeners.scrollend) > 0) {
+			return;
+		}
+
+		// Otherwise, remove the added listeners.
+		originalFn.apply(scrollPort, ['scroll', data.scrollListener]);
+		observed.delete(scrollPort);
+	}
+
+	function polyfillAddEventListener(proto) {
+		var native = proto.addEventListener;
+		proto.addEventListener = function addEventListener(type, callback) { // eslint-disable-line no-unused-vars
+			var args = global.Array.prototype.slice.apply(arguments, [0]);
+			native.apply(this, args);
+
+			if (type !== 'scroll' && type !== 'scrollend') {
+				return;
+			}
+
+			if (arguments[2] && arguments[2].once) {
+				var removerArgs = global.Array.prototype.slice.apply(arguments, [0]);
+				var _this = this;
+
+				var remover = function () {
+					_this.removeEventListener(type, remover);
+					_this.removeEventListener.apply(_this, removerArgs);
+				}
+
+				_this.addEventListener(type, remover);
+			}
+
+			args.unshift(native);
+			onAddListener.apply(this, args);
+		}
+	}
+
+	function polyfillRemoveEventListener(proto) {
+		var native = proto.removeEventListener;
+		proto.removeEventListener = function removeEventListener(type, callback) { // eslint-disable-line no-unused-vars
+			var args = global.Array.prototype.slice.apply(arguments, [0]);
+			native.apply(this, args);
+
+			if (type !== 'scroll' && type !== 'scrollend') {
+				return;
+			}
+
+			args.unshift(native);
+			onRemoveListener.apply(this, args);
+		}
+	}
+
+	polyfillAddEventListener(global.Element.prototype);
+	polyfillAddEventListener(global);
+	polyfillAddEventListener(global.document);
+	polyfillRemoveEventListener(global.Element.prototype);
+	polyfillRemoveEventListener(global);
+	polyfillRemoveEventListener(global.document);
+
+	if (!('onscrollend' in self)) {
+		// Make sure a check for 'onhashchange' in window will pass
+		global.onscrollend = null;
+	}
+}(self));