winemac: Clip surface drawn region to new visible rect on window resize.

dlls/winemac.drv/macdrv.h

@@ -141,6 +141,7 @@ extern struct window_surface *create_surface(macdrv_window window, const RECT *r
                                              struct window_surface *old_surface, BOOL use_alpha) DECLSPEC_HIDDEN;
 extern void set_window_surface(macdrv_window window, struct window_surface *window_surface) DECLSPEC_HIDDEN;
 extern void set_surface_use_alpha(struct window_surface *window_surface, BOOL use_alpha) DECLSPEC_HIDDEN;
+extern void surface_clip_to_visible_rect(struct window_surface *window_surface, const RECT *visible_rect) DECLSPEC_HIDDEN;
 
 extern void macdrv_handle_event(const macdrv_event *event) DECLSPEC_HIDDEN;
 

dlls/winemac.drv/surface.c

@@ -410,3 +410,35 @@ CGImageRef create_surface_image(void *window_surface, CGRect *rect, int copy_dat
 
     return cgimage;
 }
+
+/***********************************************************************
+ *              surface_clip_to_visible_rect
+ *
+ * Intersect the accumulated drawn region with a new visible rect,
+ * effectively discarding stale drawing in the surface slack area.
+ */
+void surface_clip_to_visible_rect(struct window_surface *window_surface, const RECT *visible_rect)
+{
+    struct macdrv_window_surface *surface = get_mac_surface(window_surface);
+
+    window_surface->funcs->lock(window_surface);
+
+    if (surface->drawn)
+    {
+        RECT rect;
+        HRGN region;
+
+        rect = *visible_rect;
+        OffsetRect(&rect, -rect.left, -rect.top);
+
+        if ((region = CreateRectRgnIndirect(&rect)))
+        {
+            CombineRgn(surface->drawn, surface->drawn, region, RGN_AND);
+            DeleteObject(region);
+
+            update_blit_data(surface);
+        }
+    }
+
+    window_surface->funcs->unlock(window_surface);
+}

dlls/winemac.drv/window.c

@@ -1284,6 +1284,7 @@ void CDECL macdrv_WindowPosChanging(HWND hwnd, HWND insert_after, UINT swp_flags
         if (!memcmp(&data->surface->rect, &surface_rect, sizeof(surface_rect)))
         {
             /* existing surface is good enough */
+            surface_clip_to_visible_rect(data->surface, visible_rect);
             window_surface_add_ref(data->surface);
             *surface = data->surface;
             goto done;