Update CSS for runner canvas and add new test cases

src/app/app.css

@@ -1081,6 +1081,7 @@ button:disabled {
 .runner-canvas {
   flex: 1 1 auto;
   cursor: grab;
+  overflow: hidden;
 }
 
 .viewport-canvas canvas,
@@ -1090,6 +1091,11 @@ button:disabled {
   height: 100%;
 }
 
+.runner-canvas canvas {
+  position: relative;
+  z-index: 1;
+}
+
 .viewport-canvas__overlay {
   position: absolute;
   bottom: 10px;
@@ -1255,9 +1261,8 @@ button:disabled {
 .runner-canvas__background-overlay {
   position: absolute;
   inset: 0;
-  z-index: 1;
+  z-index: 0;
   pointer-events: none;
-  transition: opacity 180ms linear;
 }
 
 .runner-canvas__loading-overlay--hidden {