imp: add horizontal resize feature

playground.css

@@ -18,7 +18,7 @@ body {
   display: grid;
   height: 100%;
   width: 100%;
-  grid-template-columns: 50% 50%;
+  grid-template-columns: auto 6px 100%;
 }
 
 /* LEFT BAR ****************************************/
@@ -75,6 +75,13 @@ body {
   margin-top: 8px;
 }
 
+/* SEPARATOR ****************************************/
+
+.separator {
+  background-color: #1c2128;
+  cursor: col-resize;
+}
+
 /* RIGHT PANE ****************************************/
 .right-pane {
   background-color: lightgray;
@@ -88,6 +95,10 @@ body {
   border: 0;
 }
 
+.right-pane iframe.disabled {
+  pointer-events: none;
+}
+
 .content {
   height: 100%;
 }