Bug 703787 - Crash during switch from multi to single window mode

Shells can now travel from one window to another. Canvas position code
should therefore not keep track of the image window for a specific shell
but must use whatever is the current window for this shell (even though
it may be a different one before and after the size-allocate).
This also partly fixes some positioning issue during mode switch.

app/display/gimpimagewindow.c

@@ -113,7 +113,6 @@ struct _GimpImageWindowPrivate
 
 typedef struct
 {
-  GimpImageWindow *window;
   gint             x;
   gint             y;
 } PosCorrectionData;
@@ -1468,12 +1467,12 @@ gimp_image_window_get_default_dockbook (GimpImageWindow  *window)
  * gimp_image_window_keep_canvas_pos:
  * @window:
  *
- * Stores the coordinate of the current shell image origin in
- * GtkWindow coordinates and on the first size-allocate sets the
- * offsets in the shell so the image origin remains the same in
- * GtkWindow coordinates.
+ * Stores the coordinates of the current image canvas origin relatively
+ * its GtkWindow; and on the first size-allocate sets the offsets in
+ * the shell so that the image origin remains the same (even on another
+ * GtkWindow).
  *
- * Exampe use case: The user hides docks attached to the side of image
+ * Example use case: The user hides docks attached to the side of image
  * windows. You want the image to remain fixed on the screen though,
  * so you use this function to keep the image fixed after the docks
  * have been hidden.
@@ -1501,7 +1500,6 @@ gimp_image_window_keep_canvas_pos (GimpImageWindow *window)
     {
       PosCorrectionData *data = g_new0 (PosCorrectionData, 1);
 
-      data->window = window;
       data->x      = image_origin_window_x;
       data->y      = image_origin_window_y;
 
@@ -1589,23 +1587,24 @@ gimp_image_window_shell_size_allocate (GimpDisplayShell  *shell,
                                        GtkAllocation     *allocation,
                                        PosCorrectionData *data)
 {
-  GimpImageWindow *window               = data->window;
+  GimpImageWindow *window               = gimp_display_shell_get_window (shell);
   gint             image_origin_shell_x = -1;
   gint             image_origin_shell_y = -1;
 
-  gtk_widget_translate_coordinates (GTK_WIDGET (window),
-                                    GTK_WIDGET (shell->canvas),
-                                    data->x, data->y,
-                                    &image_origin_shell_x,
-                                    &image_origin_shell_y);
-
-  /* Note that the shell offset isn't the offset of the image into the
-   * shell, but the offset of the shell relative to the image,
-   * therefore we need to negate
-   */
-  gimp_display_shell_scroll_set_offset (shell,
-                                        -image_origin_shell_x,
-                                        -image_origin_shell_y);
+  if (gtk_widget_translate_coordinates (GTK_WIDGET (window),
+                                        GTK_WIDGET (shell->canvas),
+                                        data->x, data->y,
+                                        &image_origin_shell_x,
+                                        &image_origin_shell_y))
+    {
+      /* Note that the shell offset isn't the offset of the image into the
+       * shell, but the offset of the shell relative to the image,
+       * therefore we need to negate
+       */
+      gimp_display_shell_scroll_set_offset (shell,
+                                            -image_origin_shell_x,
+                                            -image_origin_shell_y);
+    }
 
   g_signal_handlers_disconnect_by_func (shell,
                                         gimp_image_window_shell_size_allocate,