[FIXED] Previous bug fixed

src/game.js

@@ -11,17 +11,21 @@ export function init() {
 }
 
 export class Game {
+    _pageIsInView = true;
+
     constructor(width = 416, height = 416, ctx) {
         this.gameField = new GameField(width, height, ctx, canvas);
         this.gameKeyAdapter = new TankKeyAdapter(this.gameField.tank);
+
+        window.addEventListener("blur", () => { this._pageIsInView = false; });
+        window.addEventListener("focus", () => { this._pageIsInView = true; });
     }
 
     start() {
         let prevEv;
         let holding = (ev) => {
             if (!prevEv || prevEv.key != ev.key) return;
-            if (document.visibilityState == "hidden") {
-                alert("I'm hidden");
+            if (!this._pageIsInView) {
                 prevEv = undefined;
                 return;
             }