Fixed CollapsableMixin to set dimension css style to 'auto' when the transition to the expanded state is complete.

Author: Bill Schaller

src/CollapsableMixin.js

@@ -94,8 +94,12 @@ var CollapsableMixin = {
     var node = this.getCollapsableDOMNode();
 
     if (node) {
-      node.style[dimension] = this.isExpanded() ?
-        this.getCollapsableDimensionValue() + 'px' : '0px';
+        if(this.props.expanded && !this.state.collapsing) {
+            node.style[dimension] = 'auto';
+        } else {
+            node.style[dimension] = this.isExpanded() ?
+              this.getCollapsableDimensionValue() + 'px' : '0px';
+        }
     }
   },