notify popup about initialization

ext/fg/js/float.js

@@ -45,6 +45,8 @@ class DisplayFloat extends Display {
         }
 
         this.setContentScale(scale);
+
+        window.parent.postMessage('initialized', '*');
     }
 
     onError(error) {

ext/fg/js/popup.js

@@ -45,6 +45,8 @@ class Popup {
         this._container.style.height = '0px';
 
         this._updateVisibility();
+
+        window.addEventListener('message', (e) => this.onMessage(e), false);
     }
 
     // Public properties
@@ -129,6 +131,18 @@ class Popup {
         }
     }
 
+    onMessage(e) {
+        const action = e.data;
+        const handler = Popup._windowMessageHandlers.get(action);
+        if (typeof handler !== 'function') { return; }
+
+        handler(this);
+    }
+
+    setInitialized() {
+        throw new Error('Override me');
+    }
+
     // Popup-only public functions
 
     setParent(parent) {
@@ -237,7 +251,7 @@ class Popup {
                     childrenSupported: this._childrenSupported,
                     scale: this._contentScale
                 });
-                resolve();
+                this.setInitialized = resolve;
             });
             this._observeFullscreen();
             this._onFullscreenChanged();
@@ -535,4 +549,8 @@ class Popup {
     }
 }
 
+Popup._windowMessageHandlers = new Map([
+    ['initialized', (self) => self.setInitialized()]
+]);
+
 Popup.outerStylesheet = null;