Index: src/window_type.h
===================================================================
--- src/window_type.h	(revision 12656)
+++ src/window_type.h	(working copy)
@@ -7,6 +7,9 @@
 
 #include "core/enum_type.hpp"
 
+/**
+ * Window classes
+ */
 enum WindowClass {
 	WC_NONE,
 	WC_MAIN_WINDOW = WC_NONE,
@@ -93,7 +96,7 @@
 	WC_OSK,
 };
 
-struct Window;
+class Window;
 struct WindowEvent;
 typedef int32 WindowNumber;
 
Index: src/window_gui.h
===================================================================
--- src/window_gui.h	(revision 12656)
+++ src/window_gui.h	(working copy)
@@ -289,7 +289,12 @@
 /**
  * Data structure for an opened window
  */
-struct Window {
+class Window
+{
+public:
+	Window();
+	~Window();
+
 	uint16 flags4;              ///< Window flags, @see WindowFlags
 	WindowClass window_class;   ///< Window class
 	WindowNumber window_number; ///< Window number within the window class
@@ -307,7 +312,7 @@
 	byte caption_color; ///< Background color of the window caption, contains PlayerID
 
 	WindowProc *wndproc;   ///< Event handler function for the window
-	ViewPort *viewport;    ///< Pointer to viewport, if present (structure is part of derived class)
+	ViewPort *viewport;    ///< Pointer to viewport, if present
 	const Widget *original_widget; ///< Original widget layout, copied from WindowDesc
 	Widget *widget;        ///< Widgets of the window
 	uint widget_count;     ///< Number of widgets of the window
Index: src/window.cpp
===================================================================
--- src/window.cpp	(revision 12656)
+++ src/window.cpp	(working copy)
@@ -47,7 +47,14 @@
 
 byte _special_mouse_mode;
 
+Window::Window()
+{
+}
 
+Window::~Window()
+{
+}
+
 void CDECL Window::SetWidgetsDisabledState(bool disab_stat, int widgets, ...)
 {
 	va_list wdg_list;
@@ -250,34 +257,7 @@
 	}
 }
 
-static void DrawOverlappedWindow(Window* const *wz, int left, int top, int right, int bottom);
-
 /**
- * From a rectangle that needs redrawing, find the windows that intersect with the rectangle.
- * These windows should be re-painted.
- * @param left Left edge of the rectangle that should be repainted
- * @param top Top edge of the rectangle that should be repainted
- * @param right Right edge of the rectangle that should be repainted
- * @param bottom Bottom edge of the rectangle that should be repainted
- */
-void DrawOverlappedWindowForAll(int left, int top, int right, int bottom)
-{
-	Window* const *wz;
-	DrawPixelInfo bk;
-	_cur_dpi = &bk;
-
-	FOR_ALL_WINDOWS(wz) {
-		const Window *w = *wz;
-		if (right > w->left &&
-				bottom > w->top &&
-				left < w->left + w->width &&
-				top < w->top + w->height) {
-			DrawOverlappedWindow(wz, left, top, right, bottom);
-		}
-	}
-}
-
-/**
  * Generate repaint events for the visible part of window *wz within the rectangle.
  *
  * The function goes recursively upwards in the window stack, and splits the rectangle
@@ -288,8 +268,6 @@
  * @param top Top edge of the rectangle that should be repainted
  * @param right Right edge of the rectangle that should be repainted
  * @param bottom Bottom edge of the rectangle that should be repainted
- *
- * @todo Swap this function to above DrawOverlappedWindowForAll() to eliminate the forward declaration
  */
 static void DrawOverlappedWindow(Window* const *wz, int left, int top, int right, int bottom)
 {
@@ -347,6 +325,31 @@
 }
 
 /**
+ * From a rectangle that needs redrawing, find the windows that intersect with the rectangle.
+ * These windows should be re-painted.
+ * @param left Left edge of the rectangle that should be repainted
+ * @param top Top edge of the rectangle that should be repainted
+ * @param right Right edge of the rectangle that should be repainted
+ * @param bottom Bottom edge of the rectangle that should be repainted
+ */
+void DrawOverlappedWindowForAll(int left, int top, int right, int bottom)
+{
+	Window* const *wz;
+	DrawPixelInfo bk;
+	_cur_dpi = &bk;
+
+	FOR_ALL_WINDOWS(wz) {
+		const Window *w = *wz;
+		if (right > w->left &&
+				bottom > w->top &&
+				left < w->left + w->width &&
+				top < w->top + w->height) {
+			DrawOverlappedWindow(wz, left, top, right, bottom);
+		}
+	}
+}
+
+/**
  * Dispatch an event to a possibly non-existing window.
  * If the window pointer w is \c NULL, the event is not dispatched
  * @param w Window to dispatch the event to, may be \c NULL
@@ -709,8 +712,7 @@
 		DeleteWindow(w);
 	}
 
-	/* Set up window properties */
-	memset(w, 0, sizeof(*w));
+	/* Initialize all variables, and set up window properties */
 	w->window_class = cls;
 	w->flags4 = WF_WHITE_BORDER_MASK; // just opened windows have a white border
 	w->caption_color = 0xFF;
@@ -719,13 +721,44 @@
 	w->width = min_width;
 	w->height = min_height;
 	w->wndproc = proc;
-	AssignWidgetToWindow(w, widget);
+
+	w->hscroll.count = 0;
+	w->hscroll.cap = 0;
+	w->hscroll.pos = 0;
+
+	w->vscroll.count = 0;
+	w->vscroll.cap = 0;
+	w->vscroll.pos = 0;
+
+	w->vscroll2.count = 0;
+	w->vscroll2.cap = 0;
+	w->vscroll2.pos = 0;
+
 	w->resize.width = min_width;
 	w->resize.height = min_height;
 	w->resize.step_width = 1;
 	w->resize.step_height = 1;
 	w->window_number = window_number;
 
+	w->viewport = NULL;
+	w->original_widget= NULL;
+	w->widget= NULL;
+	w->widget_count = 0;
+	w->desc_flags = 0;
+
+	w->message.msg = 0;
+	w->message.wparam = 0;
+	w->message.lparam = 0;
+
+	w->parent= NULL;
+	for (int i = 0; i < WINDOW_CUSTOM_SIZE; i++) {
+		w->custom[i] = 0;
+	}
+
+	/* w->widget must be initialized before calling AssignWidgetToWindow() */
+	AssignWidgetToWindow(w, widget);
+
+
 	{
 		Window **wz = _last_z_window;
 
@@ -1053,16 +1086,21 @@
 	return NULL;
 }
 
+/**
+ * (re)initialize the windowing system
+ */
 void InitWindowSystem()
 {
 	IConsoleClose();
 
-	memset(&_windows, 0, sizeof(_windows));
 	_last_z_window = _z_windows;
 	InitViewports();
 	_no_scroll = 0;
 }
 
+/**
+ * Close down the windowing system
+ */
 void UnInitWindowSystem()
 {
 	Window **wz;
@@ -1081,6 +1119,9 @@
 	assert(_last_z_window == _z_windows);
 }
 
+/**
+ * Reset the windowing system, by means of shutting it down followed by re-initialization
+ */
 void ResetWindowSystem()
 {
 	UnInitWindowSystem();
@@ -1206,7 +1247,7 @@
 		e.event = WE_MOUSEOVER;
 		e.we.mouseover.pt.x = -1;
 		e.we.mouseover.pt.y = -1;
-		if (last_w->wndproc) last_w->wndproc(last_w, &e);
+		if (last_w->wndproc != NULL) last_w->wndproc(last_w, &e);
 	}
 	last_w = w;