automatically resize sandbox iframes

static/css/main.css

@@ -574,6 +574,7 @@ th-literate-program[data-mode="graphics"] {
     & iframe {
         border-style: none;
         border-radius: 4px;
+        display: block;
     }
 
     /* The inner iframe is hidden until something requests display. */

static/js/sandbox.js

@@ -6,11 +6,5 @@ export class Sketch {
         this.ctx = this.canvas.getContext("2d");
 
         document.body.appendChild(this.canvas);
-
-        postMessage({
-            kind: "resize",
-            width,
-            height,
-        });
     }
 }