app: make new images jump around much less

Implement a mechanism to suspend/resume GimpImageWindow's "keep canvas
pos" logic which is used to keep the image in place across widget
changed such as show/hide rulers.

gimp_display_shell_fill(): call suspend()/resume() around
gimp_display_shell_appearance_update(), and center the image after
calculating the initial scale factor, so the image jumping at least
starts at the approximately right position.

app/display/gimpdisplayshell.c

@@ -1519,9 +1519,17 @@ gimp_display_shell_fill (GimpDisplayShell *shell,
   gimp_display_shell_set_unit (shell, unit);
   gimp_display_shell_set_initial_scale (shell, scale, NULL, NULL);
 
+  /* 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, shell->display->config);
 
+  gimp_image_window_suspend_keep_pos (window);
   gimp_display_shell_appearance_update (shell);
+  gimp_image_window_resume_keep_pos (window);
+
   gimp_image_window_update_tabs (window);
 #if 0
   gimp_help_set_help_data (shell->canvas, NULL, NULL);

app/display/gimpimagewindow.c

@@ -118,6 +118,8 @@ struct _GimpImageWindowPrivate
 
   GdkScreen         *initial_screen;
   gint               initial_monitor;
+
+  gint               suspend_keep_pos;
 };
 
 typedef struct
@@ -1599,12 +1601,22 @@ gimp_image_window_get_default_dockbook (GimpImageWindow  *window)
 void
 gimp_image_window_keep_canvas_pos (GimpImageWindow *window)
 {
-  GimpDisplayShell  *shell = gimp_image_window_get_active_shell (window);
+  GimpImageWindowPrivate *private;
+  GimpDisplayShell       *shell;
   gint                    canvas_x;
   gint                    canvas_y;
   gint                    window_x;
   gint                    window_y;
 
+  g_return_if_fail (GIMP_IS_IMAGE_WINDOW (window));
+
+  private = GIMP_IMAGE_WINDOW_GET_PRIVATE (window);
+
+  if (private->suspend_keep_pos > 0)
+    return;
+
+  shell = gimp_image_window_get_active_shell (window);
+
   gimp_display_shell_transform_xy (shell, 0.0, 0.0, &canvas_x, &canvas_y);
 
   if (gtk_widget_translate_coordinates (GTK_WIDGET (shell->canvas),
@@ -1626,6 +1638,31 @@ gimp_image_window_keep_canvas_pos (GimpImageWindow *window)
     }
 }
 
+void
+gimp_image_window_suspend_keep_pos (GimpImageWindow  *window)
+{
+  GimpImageWindowPrivate *private;
+
+  g_return_if_fail (GIMP_IS_IMAGE_WINDOW (window));
+
+  private = GIMP_IMAGE_WINDOW_GET_PRIVATE (window);
+
+  private->suspend_keep_pos++;
+}
+
+void
+gimp_image_window_resume_keep_pos (GimpImageWindow  *window)
+{
+  GimpImageWindowPrivate *private;
+
+  g_return_if_fail (GIMP_IS_IMAGE_WINDOW (window));
+
+  private = GIMP_IMAGE_WINDOW_GET_PRIVATE (window);
+
+  g_return_if_fail (private->suspend_keep_pos > 0);
+
+  private->suspend_keep_pos--;
+}
 
 /**
  * gimp_image_window_update_tabs:
@@ -1803,9 +1840,11 @@ gimp_image_window_shell_size_allocate (GimpDisplayShell  *shell,
                                         data->canvas_x, data->canvas_y,
                                         &new_window_x, &new_window_y))
     {
-      gimp_display_shell_scroll (shell,
+      gint off_x = new_window_x - data->window_x;
-                                 new_window_x - data->window_x,
+      gint off_y = new_window_y - data->window_y;
-                                 new_window_y - data->window_y);
+
+      if (off_x || off_y)
+        gimp_display_shell_scroll (shell, off_x, off_y);
     }
 
   g_signal_handlers_disconnect_by_func (shell,

app/display/gimpimagewindow.h

@@ -93,6 +93,8 @@ void               gimp_image_window_shrink_wrap          (GimpImageWindow  *win
 GtkWidget        * gimp_image_window_get_default_dockbook (GimpImageWindow  *window);
 
 void               gimp_image_window_keep_canvas_pos      (GimpImageWindow  *window);
+void               gimp_image_window_suspend_keep_pos     (GimpImageWindow  *window);
+void               gimp_image_window_resume_keep_pos      (GimpImageWindow  *window);
 
 void               gimp_image_window_update_tabs          (GimpImageWindow  *window);