Steps to Reproduce

run fiddle
http://zkfiddle.org/sample/1ho88ob/1-hidden-tab-tree-initpadsizes

Go to the "tree" tab
click the add button 3 times to add some tree items
Go to the the "buttons" tab
Click on the add button 3 times to add some tree items
Go to the "Tree" tab again

Current Result

tree is shrunk to 2 treeitems
additional clicks on the add button will cause the tree to resize, then will cause the tree to add missing items

Expected Result

tree initPadSize calculation should not fail offscreen

Debug Info

When the dom node is hidden, calculation fails due to missing height information.
Will affect any case where a tree is updated while hidden.

Root Cause

in the _initPadSizes function, b.offsetHeight == 0 if the dom node is hidden

Workaround

			<script><![CDATA[  
				zk.afterLoad('zul.sel', function() {
					var xTree = {};
					zk.override(zul.sel.Tree.prototype, xTree ,{
						onShow : function() {
							this._initPadSizes();
						},
						bind_: function(){
							var result = xTree.bind_.apply(this, arguments);
							zWatch.listen({onShow: this});
						},
						unbind_: function(){
							zWatch.unlisten({onShow: this});
							var result = xTree.unbind_.apply(this, arguments);
						}
					});//zk.override
				});//zk.afterLoad
			]]></script>