Steps to Reproduce

Try to override _listenFlex, _unlistenFlex:

 	<script><![CDATA[ 
	zk.afterLoad("zk", function () {
	    var _x_ = {};
	    zk.override(zk.widget._, _x_, {
	    	_listenFlex(wgt) {

Current Result

Replaces the publicly visible reference to zk.widget._,listenFlex, but is not invoked by zk.Widget.prototype.bind or setHflex_, or other consumers, because they are using a direct reference to the original function, not a reference to the function located in the public member:
https://github.com/zkoss/zk/blob/7185169f3cb3814a8f9b7c7d3d9bde265c19a22f/zk/src/main/resources/web/js/zk/widget.ts#L1581-L1583
https://github.com/zkoss/zk/blob/7185169f3cb3814a8f9b7c7d3d9bde265c19a22f/zk/src/main/resources/web/js/zk/widget.ts#L1604-L1607
https://github.com/zkoss/zk/blob/7185169f3cb3814a8f9b7c7d3d9bde265c19a22f/zk/src/main/resources/web/js/zk/widget.ts#L3902-L3904

As a result, the only way to override listenFlex and _unlistenFlex is to override all consumers to replace the direct reference to _listenFlex by a reference to the publicly available zk.widget.._listenFlex

Expected Result

Overriding public members should cause the override to be used by consumers, not the unmodified original function

Debug Information

• Several class-level members are referred to directly by functions. This makes overrides difficult, as all consumers have to be replaced.
• Another example found during the same patch:
  zk.Widget.prototype.setHflex makes a reference to zk.widget._.binds directly as _binds, which makes overriding the function more difficult, same issue.
• please solve similar bugs in other widgets

Workaround

replace the entire set of functions using the class-level values