You can try use dojox.layout.ExpandoPane at your left pane.
var bc = new BorderContainer({
splitter:true,
gutters:false
}, containerDiv);
new ExpandoPane({
region:"left",
title: "Expandable pane",
className: yourClassName
}).placeAt(bc);
EDIT: update based on request:
put this in your head:
<link href="js/dojox/layout/resources/ExpandoPane.css" rel="stylesheet" type="text/css" />
change you pane like this:
<div id="leftCol" class="edgePanel" title="Left Expando" data-dojo-type="dojox/layout/ExpandoPane" data-dojo-props="title:'Left Expando', region: 'leading', splitter: true" style="width: 250px;"> <div data-dojo-type="dijit/layout/TabContainer" style="width: 400px; height: 95%;" tabstrip="true"> <div data-dojo-type="dijit/layout/ContentPane" title="Layers" selected="true"> <div id="CheckboxTree"> </div> </div> </div> </div>