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)
@@ -22,8 +22,8 @@
 
 #include "table/sprites.h"
 
-/** delta between mouse cursor and upper left corner of dragged window */
-static Point _drag_delta;
+static Point _drag_delta; //< delta between mouse cursor and upper left corner of dragged window
+static Window *_mouseover_last_w = NULL; ///< Window of the last MOUSEOVER event
 
 static Window _windows[MAX_NUMBER_OF_WINDOWS];
 
@@ -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
@@ -430,6 +433,13 @@
 	w->widget_count = 0;
 	w->parent = NULL;
 
+	if (_mouseover_last_w == w) {
+		/* Prevent Mouseover() from resetting mouse-over coordinates on a
+		 * non-existing window
+		 */
+		_mouseover_last_w = NULL;
+	}
+
 	/* Find the window in the z-array, and effectively remove it
 	 * by moving all windows after it one to the left */
 	Window **wz = FindWindowZPosition(w);
@@ -719,6 +729,7 @@
 	w->width = min_width;
 	w->height = min_height;
 	w->wndproc = proc;
+	w->widget = NULL; // w->widget must be initialized before calling AssignWidgetToWindow()
 	AssignWidgetToWindow(w, widget);
 	w->resize.width = min_width;
 	w->resize.height = min_height;
@@ -1053,6 +1064,9 @@
 	return NULL;
 }
 
+/**
+ * (re)initialize the windowing system
+ */
 void InitWindowSystem()
 {
 	IConsoleClose();
@@ -1063,6 +1077,9 @@
 	_no_scroll = 0;
 }
 
+/**
+ * Close down the windowing system
+ */
 void UnInitWindowSystem()
 {
 	Window **wz;
@@ -1081,6 +1098,9 @@
 	assert(_last_z_window == _z_windows);
 }
 
+/**
+ * Reset the windowing system, by means of shutting it down followed by re-initialization
+ */
 void ResetWindowSystem()
 {
 	UnInitWindowSystem();
@@ -1197,18 +1217,21 @@
 {
 	Window *w;
 	WindowEvent e;
-	static Window *last_w = NULL;
 
 	w = FindWindowFromPt(_cursor.pos.x, _cursor.pos.y);
 
 	/* We changed window, put a MOUSEOVER event to the last window */
-	if (last_w != NULL && last_w != w) {
+	if (_mouseover_last_w != NULL && _mouseover_last_w != w) {
+		/* Reset mouse-over coordinates of previous window */
 		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 (_mouseover_last_w->wndproc != NULL) {
+			_mouseover_last_w->wndproc(_mouseover_last_w, &e);
+		}
 	}
-	last_w = w;
+	_mouseover_last_w = w;
+	/* In DeleteWindow(), _mouseover_last_w will get reset when the window is deleted */
 
 	if (w != NULL) {
 		/* send an event in client coordinates. */