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This occurs when the openmsx emulator is running full screen and the user reverts to the desktop by hitting Win+D.

In this case, the screen size reverts to the original size (in my case, 1920x1200) instead of 1024x768.  However, in this scenario, the SDL window is now off-screen, and Windows returns some rather odd values for the physical position of the middle of the window (e.g. center.x = -31520, center.y = -31640).

Long story short, we created an SDL patch for openmsx builds that causes DIB_GenerateMouseMotionEvent to only post a new event if (after calling SetCursorPos) the cursor's position actually changed from the original position (in the scenario I describe, it actually doesn't:  it was 0,0 before and it remains 0,0 after calling SetCursorPos).

In other words:

diff -ru SDL-1.2.14.org/src/video/windib/SDL_dibevents.c SDL-1.2.14/src/video/windib/SDL_dibevents.c
--- SDL-1.2.14.org/src/video/windib/SDL_dibevents.c	Mon Feb 15 11:49:09 2010
+++ SDL-1.2.14/src/video/windib/SDL_dibevents.c	Mon Feb 15 11:51:15 2010
@@ -328,12 +328,13 @@
 	extern int mouse_relative;
 	extern int posted;
 
-	POINT mouse;
+	POINT mouse, originalMouse;
 #ifdef _WIN32_WCE
 	if ( !GetCursorPos(&mouse) && !GetLastStylusPos(&mouse) ) return;
 #else
 	if ( !GetCursorPos(&mouse) ) return;
 #endif
+	originalMouse = mouse;
 
 	if ( mouse_relative ) {
 		POINT center;
@@ -345,7 +346,11 @@
 		mouse.y -= center.y;
 		if ( mouse.x || mouse.y ) {
 			SetCursorPos(center.x, center.y);
-			posted = SDL_PrivateMouseMotion(0, 1, (Sint16)mouse.x, (Sint16)mouse.y);
+			GetCursorPos(&mouse);
+			// Make sure the mouse position actually changed
+			if (mouse.x != originalMouse.x || mouse.y != originalMouse.y) {
+				posted = SDL_PrivateMouseMotion(0, 1, (Sint16)mouse.x, (Sint16)mouse.y);
+			}
 		}
 	} else {
 		ScreenToClient(SDL_Window, &mouse);