flash background when 'run' is clicked.

runestone/activecode/css/activecode.css

@@ -116,3 +116,16 @@
 .ac-disabled {
 	pointer-events: none;
 }
+
+.ac_output pre.ac-code-changed {
+	background: initial;
+	animation: flash-background 1s;
+}
+@keyframes flash-background {
+	0% {
+		background: rgba(255, 255, 0, 0.3);
+	}
+	100% {
+		background: initial;
+	}
+}

runestone/activecode/js/activecode.js

@@ -671,10 +671,22 @@ ActiveCode.prototype.buildProg = function() {
     return prog;
 };
 
+ActiveCode.prototype.flashBackground = function() {
+  // Provide a visual cue that the code was re-run, as I was getting feedback
+  // that, when restructuring code that shouldn't have changed output, they
+  // were worried about whether the program had actually run again.
+  this.output.classList.remove('ac-code-changed');
+  // reflow to ensure that the class removal is synced to DOM
+  void this.output.offsetWidth;
+  // add .ac-code-changed to trigger background animation
+  this.output.classList.add('ac-code-changed');
+};
+
 ActiveCode.prototype.runProg = function() {
         var prog = this.buildProg();
 
         $(this.output).text('');
+        this.flashBackground();
 
         $(this.eContainer).remove();
         Sk.configure({output : this.outputfun.bind(this),