diff --git a/src/window.cpp b/src/window.cpp
index 95dcce7a00..5e9d22e1b2 100644
--- a/src/window.cpp
+++ b/src/window.cpp
@@ -491,6 +491,7 @@ void Window::ReInit(int rx, int ry)
 	int window_width  = this->width;
 	int window_height = this->height;
 
+	this->OnInit();
 	/* Re-initialize the window from the ground up. No need to change the nested_array, as all widgets stay where they are. */
 	this->nested_root->SetupSmallestSize(this, false);
 	this->nested_root->AssignSizePosition(ST_SMALLEST, 0, 0, this->nested_root->smallest_x, this->nested_root->smallest_y, _dynlang.text_dir == TD_RTL);
@@ -786,7 +787,8 @@ void Window::InitializeData(WindowClass cls, int window_number, uint32 desc_flag
 	this->window_number = window_number;
 	this->desc_flags = desc_flags;
 
-	/* If available, initialize nested widget tree. */
+	this->OnInit();
+	/* Initialize nested widget tree. */
 	if (this->nested_array == NULL) {
 		this->nested_array = CallocT<NWidgetBase *>(this->nested_array_size);
 		this->nested_root->SetupSmallestSize(this, true);
diff --git a/src/window_gui.h b/src/window_gui.h
index c72b99569a..4e327a3c41 100644
--- a/src/window_gui.h
+++ b/src/window_gui.h
@@ -534,6 +534,12 @@ public:
 
 	/*** Event handling ***/
 
+	/**
+	 * Notification that the nested widget tree gets initialized. The event can be used to perform general computations.
+	 * @note #nested_root and/or #nested_array (normally accessed via #GetWidget()) may not exist during this call.
+	 */
+	virtual void OnInit() { }
+
 	/**
 	 * Compute the initial position of the window.
 	 * @param *desc         The pointer to the WindowDesc of the window to create.