Ensure correct behaviour of Esc key in overlays

package.json

@@ -1,6 +1,6 @@
 {
     "name": "minesweeper",
-    "version": "0.82.14",
+    "version": "0.82.15",
     "description": "Just Minesweeper!",
     "author": "Florine W. Dekker",
     "browser": "dist/bundle.js",

src/main/js/Common.ts

@@ -3,7 +3,7 @@ import alea from "alea";
 
 
 /**
- * Blurs the currently active element, if possible.
+ * Removes focus from the currently active element, if possible.
  */
 export function blurActiveElement(): void {
     // @ts-ignore

src/main/js/UI.ts

@@ -82,6 +82,6 @@ export class Overlay {
      * Returns `true` if and only if this overlay is currently visible.
      */
     isVisible(): boolean {
-        return this.overlay.classList.contains("hidden");
+        return !this.overlay.classList.contains("hidden");
     }
 }