Index: intern/ghost/intern/GHOST_SystemWin32.cpp
===================================================================
--- intern/ghost/intern/GHOST_SystemWin32.cpp	(Revision 20165)
+++ intern/ghost/intern/GHOST_SystemWin32.cpp	(Arbeitskopie)
@@ -766,6 +766,7 @@
 					 * message without calling DefWindowProc.
 					 */
 					event = processWindowEvent(GHOST_kEventWindowSize, window);
+					break;
 				case WM_CAPTURECHANGED:
 					window->lostMouseCapture();
 					break;
@@ -904,7 +905,12 @@
 
 	if (event) {
 		system->pushEvent(event);
-		lResult = 0;
+		/* WARNING: Let DefWindowProc handle WM_ACTIVATE, otherwise WM_MOUSEWHEEL
+		will not be dispatched to OUR active window if we minimize one of OUR windows. */
+		if (msg == WM_ACTIVATE) 
+			lResult = ::DefWindowProc(hwnd, msg, wParam, lParam);
+		else 
+			lResult = 0;
 	}
 	else {
 		lResult = ::DefWindowProc(hwnd, msg, wParam, lParam);