Steps to Reproduce

run fiddle:
http://zkfiddle.org/sample/1gsdtbq/1-Splitter-no-onchanging
click on arrows up or down on the first spinner

Current Result

onChanging is not called clicks.
onChanging is evaluated during blur

Expected Result

onChanging should be called for each click

Debug Info

Most components will perform some variation of

this.proxy(inputWidget._onChanging)();

when triggering onChanging (usually inside a timeout)
Spinner calls onChanging directly during _btnUp
https://github.com/zkoss/zk/blob/884e074c3ab999798dcf38b01564ec1f4c940818/zul/src/archive/web/js/zul/inp/Spinner.js#L158

which then in turn invoke the static _onChanging here:
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/inp/InputWidget.js#L424

in this scenario, when the actual onChanging function is called here:
https://github.com/zkoss/zk/blob/master/zul/src/archive/web/js/zul/inp/InputWidget.js#L941
the 'this' object doesn't refer to the spinner wgt (as it would in a proxied method) but to the function object instead. since this is invalid, _onChanging fails.

Root Cause

Change of architecture
https://github.com/zkoss/zk/commit/3c05a5010116e6cd70a751daf2735206cd9487b3

Workaround

	<script><![CDATA[
		zk.afterLoad('zul.inp', function() {
			var xSpinner = {};
			zk.override(zul.inp.Spinner.prototype, xSpinner ,{
				_btnUp  : function() {
					//before
					var result = xSpinner._btnUp .apply(this, arguments);
					//after
					var inputWidget = zul.inp.InputWidget;
					inputWidget._stopOnChanging(this);
					this.proxy(inputWidget._onChanging)();
					return result;
				}
			});//zk.override
		});//zk.afterLoad
	]]></script>