app: fix jumping around of newly created images

Put the center_image_on_size_allocate() code into the canvas'
size-allocate callbacck.

As a side effect we now have a flag in GimpDisplayShell which
indicates that there will be a size allocate before the next frame, so
simply skip drawing the canvas completely. This fixes new images
jumping around when they are first shown.

(cherry picked from commit c0480f502d)

(this fix is actually a side effect from fixing something else in
master)

app/display/gimpdisplayshell-callbacks.c

@@ -235,9 +235,15 @@ gimp_display_shell_canvas_size_allocate (GtkWidget        *widget,
       gimp_display_shell_scroll_clamp_and_update (shell);
       gimp_display_shell_scaled (shell);
 
-      /* Reset */
       shell->size_allocate_from_configure_event = FALSE;
     }
+
+  if (shell->size_allocate_center_image)
+    {
+      gimp_display_shell_scroll_center_image (shell, TRUE, TRUE);
+
+      shell->size_allocate_center_image = FALSE;
+    }
 }
 
 gboolean
@@ -249,6 +255,12 @@ gimp_display_shell_canvas_expose (GtkWidget        *widget,
   if (! shell->display || ! gimp_display_get_shell (shell->display))
     return TRUE;
 
+  /*  we will scroll around in the next tick anyway, so we just can as
+   *  well skip the drawing of this frame and wait for the next
+   */
+  if (shell->size_allocate_center_image)
+    return TRUE;
+
   /*  ignore events on overlays  */
   if (eevent->window == gtk_widget_get_window (widget))
     {

app/display/gimpdisplayshell-scroll.c

@@ -389,60 +389,6 @@ gimp_display_shell_scroll_center_image (GimpDisplayShell *shell,
   gimp_display_shell_scroll (shell, offset_x, offset_y);
 }
 
-typedef struct
-{
-  GimpDisplayShell *shell;
-  gboolean          vertically;
-  gboolean          horizontally;
-} CenterImageData;
-
-static void
-gimp_display_shell_scroll_center_image_callback (GtkWidget       *canvas,
-                                                 GtkAllocation   *allocation,
-                                                 CenterImageData *data)
-{
-  g_signal_handlers_disconnect_by_func (canvas,
-                                        gimp_display_shell_scroll_center_image_callback,
-                                        data);
-
-  gimp_display_shell_scroll_center_image (data->shell,
-                                          data->horizontally,
-                                          data->vertically);
-
-  g_slice_free (CenterImageData, data);
-}
-
-/**
- * gimp_display_shell_scroll_center_image_on_size_allocate:
- * @shell:
- *
- * Centers the image in the display as soon as the canvas has got its
- * new size.
- *
- * Only call this if you are sure the canvas size will change.
- * (Otherwise the signal connection and centering will lurk until the
- * canvas size is changed e.g. by toggling the rulers.)
- **/
-void
-gimp_display_shell_scroll_center_image_on_size_allocate (GimpDisplayShell *shell,
-                                                         gboolean          horizontally,
-                                                         gboolean          vertically)
-{
-  CenterImageData *data;
-
-  g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
-
-  data = g_slice_new (CenterImageData);
-
-  data->shell        = shell;
-  data->horizontally = horizontally;
-  data->vertically   = vertically;
-
-  g_signal_connect (shell->canvas, "size-allocate",
-                    G_CALLBACK (gimp_display_shell_scroll_center_image_callback),
-                    data);
-}
-
 /**
  * gimp_display_shell_scroll_get_scaled_viewport:
  * @shell:

app/display/gimpdisplayshell-scroll.h

@@ -40,10 +40,6 @@ void   gimp_display_shell_scroll_center_image_xy     (GimpDisplayShell *shell,
 void   gimp_display_shell_scroll_center_image        (GimpDisplayShell *shell,
                                                       gboolean          horizontally,
                                                       gboolean          vertically);
-void   gimp_display_shell_scroll_center_image_on_size_allocate
-                                                     (GimpDisplayShell *shell,
-                                                      gboolean          horizontally,
-                                                      gboolean          vertically);
 
 void   gimp_display_shell_scroll_get_scaled_viewport (GimpDisplayShell *shell,
                                                       gint             *x,

app/display/gimpdisplayshell.c

@@ -779,8 +779,7 @@ gimp_display_shell_constructed (GObject *object)
        * not even finished creating the display shell, we can safely
        * assume we will get a size-allocate later.
        */
-      gimp_display_shell_scroll_center_image_on_size_allocate (shell,
-                                                               TRUE, TRUE);
+      shell->size_allocate_center_image = TRUE;
     }
   else
     {
@@ -1505,11 +1504,6 @@ gimp_display_shell_fill (GimpDisplayShell *shell,
   gimp_display_shell_set_initial_scale (shell, scale, NULL, NULL);
   gimp_display_shell_scale_update (shell);
 
-  /* center the image so subsequent stuff only moves it a little in
-   * the center
-   */
-  gimp_display_shell_scroll_center_image (shell, TRUE, TRUE);
-
   gimp_display_shell_sync_config (shell, config);
 
   gimp_image_window_suspend_keep_pos (window);
@@ -1526,7 +1520,7 @@ gimp_display_shell_fill (GimpDisplayShell *shell,
   /* A size-allocate will always occur because the scrollbars will
    * become visible forcing the canvas to become smaller
    */
-  gimp_display_shell_scroll_center_image_on_size_allocate (shell, TRUE, TRUE);
+  shell->size_allocate_center_image = TRUE;
 
   if (shell->blink_timeout_id)
     {

app/display/gimpdisplayshell.h

@@ -179,6 +179,7 @@ struct _GimpDisplayShell
   gboolean           zoom_on_resize;
 
   gboolean           size_allocate_from_configure_event;
+  gboolean           size_allocate_center_image;
 
   /*  the state of gimp_display_shell_tool_events()  */
   gboolean           pointer_grabbed;