Fix #550. Use min-width: 0 after flexbox orientation switch.

By default, flexbox items have a min size of -webkit-min-content in the
the flex axis. I previously switched the pane item-views to flex
horizontally for performance reasons (preventing over-aggressive
repaints), but forgot to change min-height to min-width.

static/root-view.less

@@ -62,7 +62,7 @@ html, body {
 
   .pane .item-views > * {
     -webkit-flex: 1;
-    min-height: 0;
+    min-width: 0;
   }
 }