MouseManager.isTop is a new boolean read-only property that flags if the mouse event listeners were attached to window.top (true), or just window (false). By default Phaser will attempt window.top, but this isn't possible in all environments, such as cross-origin iframes, so it will fall back to window in those cases and set this property to false

Author: photonstorm

src/input/mouse/MouseManager.js

@@ -202,6 +202,20 @@ var MouseManager = new Class({
          */
         this.pointerLockChange = NOOP;
 
+        /**
+         * Are the event listeners hooked into `window.top` or `window`?
+         *
+         * This is set during the `boot` sequence. If the browser does not have access to `window.top`,
+         * such as in cross-origin iframe environments, this property gets set to `false` and the events
+         * are hooked into `window` instead.
+         *
+         * @name Phaser.Input.Mouse.MouseManager#isTop
+         * @type {boolean}
+         * @readonly
+         * @since 3.50.0
+         */
+        this.isTop = true;
+
         inputManager.events.once(InputEvents.MANAGER_BOOT, this.boot, this);
     },
 
@@ -436,8 +450,18 @@ var MouseManager = new Class({
 
         if (window && manager.game.config.inputWindowEvents)
         {
-            window.top.addEventListener('mousedown', this.onMouseDownWindow, passive);
-            window.top.addEventListener('mouseup', this.onMouseUpWindow, passive);
+            try
+            {
+                window.top.addEventListener('mousedown', this.onMouseDownWindow, passive);
+                window.top.addEventListener('mouseup', this.onMouseUpWindow, passive);
+            }
+            catch (exception)
+            {
+                window.addEventListener('mousedown', this.onMouseDownWindow, passive);
+                window.addEventListener('mouseup', this.onMouseUpWindow, passive);
+
+                this.isTop = false;
+            }
         }
 
         if (Features.pointerLock)
@@ -478,8 +502,10 @@ var MouseManager = new Class({
 
         if (window)
         {
-            window.top.removeEventListener('mousedown', this.onMouseDownWindow);
-            window.top.removeEventListener('mouseup', this.onMouseUpWindow);
+            target = (this.isTop) ? window.top : window;
+
+            target.removeEventListener('mousedown', this.onMouseDownWindow);
+            target.removeEventListener('mouseup', this.onMouseUpWindow);
         }
 
         if (Features.pointerLock)