Selections don't absorb click events

Clicking on a selection was causing the click event not to reach the editor, which was causing the editor to lose focus. Using CSS to disallow pointer events on selection region elements fixes this.

static/editor.css

@@ -107,4 +107,5 @@
   position: absolute;
   background: white;
   opacity: .25;
+  pointer-events: none;
 }