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Date: Tue, 15 Feb 2005 21:28:48 +0900 (JST)

From: "Michael Leonhard"
Subject: [SDL] resize bug on Win32 and patch

This is my first post to this mailing list.  In this email I will detail a
bug in the behavior of resizable SDL windows on Win32.  Then I will
explain the solution and provide a patch.


Under Windows, an SDL display created with the SDL_RESIZABLE flag exhibits
quirky behavior when being maximized.  The window is resized to the proper
size, but it is shifted upwards about half the height of the title bar.
Similarly, a window whose origin is above the top of the screen will
spontaneously move its upper-left origin upon being resized.  After two
such resize-induced moves, the title bar will be entirely off the top edge
of the screen.  Subsequently, when the mouse is clicked and released on
the window border, the window will shrink its height spontaneously.  This
height shrinkage occurs even if the user did not resize the border.

To observe this curious situation, please invoke:
SDL-1.2.8/test/testwm.exe -resize


A pair of integers, SDL_windowX and SDL_windowY, are defined in
video/wincommon/SDL_sysevents.c.  They are used by the DirectX video
driver and the DIB video driver:
As I understand the source code, the primary use of these variables is to
create a rectangle that represents the surface area in CLIENT SPACE.
Client space refers to a coordinate system that originates at the upper
left corner of a Win32 Window's drawable area.  This is just inside the
window border and title bar.  This client space rectangle, called bounds,
is subsequently converted to screen space with a call to
AdjustWindowRectEx.  The problem is found in SDL's handling of the
WM_WINDOWPOSCHANGED message.  According to MSDN,

  "The WM_WINDOWPOSCHANGED message is sent to a window whose
   size, position, or place in the Z order has changed as a
   result of a call to the SetWindowPos function or another
   window-management function."

I have confirmed that this message is indeed being sent to the SDL window
when the mouse is clicked on the window border, even if the window border
is not dragged.

In video/wincommon/SDL_sysevents.c, on line 464, in response to the
WM_WINDOWPOSCHANGED message, the (potentially) new client rectangle is
obtained.  This rectangle is translated into screen coordinates and THEN
assigned to the SDL_windowX and Y variables.  Thus screen coordinates are
being assigned to client coordinate variables.  Once this is understood,
the solution is apparent:  assign SDL_windowX and Y before translating the
rectangle to screen coordinates.  This is accomplished by the following

  • Loading branch information
slouken committed Jan 29, 2006
1 parent b60b9f0 commit 56f650eed5bc1b4b5d1d60509cc36fb23148dea4
Showing with 4 additions and 6 deletions.
  1. +4 −6 src/video/wincommon/SDL_sysevents.c
@@ -565,15 +565,13 @@ LONG CALLBACK WinMessage(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
int w, h;

GetClientRect(SDL_Window, &SDL_bounds);
ClientToScreen(SDL_Window, (LPPOINT)&SDL_bounds);
ClientToScreen(SDL_Window, (LPPOINT)&SDL_bounds+1);
if ( SDL_bounds.left || ) {
SDL_windowX = SDL_bounds.left;
SDL_windowY =;
SDL_windowX = SDL_bounds.left;
SDL_windowY =;
w = SDL_bounds.right-SDL_bounds.left;
h =;
if ( this->input_grab != SDL_GRAB_OFF ) {
ClientToScreen(SDL_Window, (LPPOINT)&SDL_bounds);
ClientToScreen(SDL_Window, (LPPOINT)&SDL_bounds+1);
if ( SDL_PublicSurface &&

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