Fix #949 layer list in viewer not scrollable

app/static/app/js/components/LayersControlLayer.jsx

@@ -274,7 +274,7 @@ export default class LayersControlLayer extends React.Component {
 
         {this.state.expanded ? 
         <div className="layer-expanded">
-            <Histogram width={280}
+            <Histogram width={274}
                         loading={histogramLoading}
                         statistics={tmeta.statistics} 
                         colorMap={cmapValues}

app/static/app/js/css/LayersControlPanel.scss

@@ -3,6 +3,7 @@
   background: #fff;
   width: 300px;
   min-height: 74px;
+  max-height: 400px;
   overflow-y: auto;
 
   .loading{