Don't access GtkAdjustment's members directly

but also don't use its accessors because doing that would emit
"changed" multiple times when setting up an adjustment with multiple
utility functions. Instead, use g_object_set() and freeze/thaw
notification around all calls. g_object_thaw_notify() will make sure
"changed" is emitted if anything has changed since freezing.

app/display/gimpdisplayshell-callbacks.c

@@ -2007,9 +2007,11 @@ gimp_display_shell_hscrollbar_update_range (GtkRange         *range,
       (scroll == GTK_SCROLL_PAGE_FORWARD))
     return FALSE;
 
+  g_object_freeze_notify (G_OBJECT (shell->hsbdata));
+
   gimp_display_shell_scroll_setup_hscrollbar (shell, value);
 
-  gtk_adjustment_changed (shell->hsbdata);
+  g_object_thaw_notify (G_OBJECT (shell->hsbdata)); /* emits "changed" */
 
   return FALSE;
 }
@@ -2028,9 +2030,11 @@ gimp_display_shell_vscrollbar_update_range (GtkRange         *range,
       (scroll == GTK_SCROLL_PAGE_FORWARD))
     return FALSE;
 
+  g_object_freeze_notify (G_OBJECT (shell->vsbdata));
+
   gimp_display_shell_scroll_setup_vscrollbar (shell, value);
 
-  gtk_adjustment_changed (shell->vsbdata);
+  g_object_thaw_notify (G_OBJECT (shell->vsbdata)); /* emits "changed" */
 
   return FALSE;
 }

app/display/gimpdisplayshell-scale.c

@@ -134,24 +134,32 @@ gimp_display_shell_scale_update_scrollbars (GimpDisplayShell *shell)
 
   /* Horizontal scrollbar */
 
-  shell->hsbdata->value          = shell->offset_x;
+  g_object_freeze_notify (G_OBJECT (shell->hsbdata));
-  shell->hsbdata->page_size      = shell->disp_width;
+
-  shell->hsbdata->page_increment = shell->disp_width / 2;
+  g_object_set (shell->hsbdata,
+                "value",          (gdouble) shell->offset_x,
+                "page-size",      (gdouble) shell->disp_width,
+                "page-increment", (gdouble) shell->disp_width / 2,
+                NULL);
 
   gimp_display_shell_scroll_setup_hscrollbar (shell, shell->offset_x);
 
-  gtk_adjustment_changed (shell->hsbdata);
+  g_object_thaw_notify (G_OBJECT (shell->hsbdata)); /* emits "changed" */
 
 
   /* Vertcal scrollbar */
 
-  shell->vsbdata->value          = shell->offset_y;
+  g_object_freeze_notify (G_OBJECT (shell->vsbdata));
-  shell->vsbdata->page_size      = shell->disp_height;
+
-  shell->vsbdata->page_increment = shell->disp_height / 2;
+  g_object_set (shell->vsbdata,
+                "value",          (gdouble) shell->offset_y,
+                "page-size",      (gdouble) shell->disp_height,
+                "page-increment", (gdouble) shell->disp_height / 2,
+                NULL);
 
   gimp_display_shell_scroll_setup_vscrollbar (shell, shell->offset_y);
 
-  gtk_adjustment_changed (shell->vsbdata);
+  g_object_thaw_notify (G_OBJECT (shell->vsbdata)); /* emits "changed" */
 }
 
 /**

app/display/gimpdisplayshell-scroll.c

@@ -546,7 +546,9 @@ void
 gimp_display_shell_scroll_setup_hscrollbar (GimpDisplayShell *shell,
                                             gdouble           value)
 {
-  gint sw;
+  gint    sw;
+  gdouble lower;
+  gdouble upper;
 
   g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
 
@@ -558,22 +560,22 @@ gimp_display_shell_scroll_setup_hscrollbar (GimpDisplayShell *shell,
 
   if (shell->disp_width < sw)
     {
-      shell->hsbdata->lower = MIN (value,
+      lower = MIN (value, 0);
-                                   0);
+      upper = MAX (value + shell->disp_width, sw);
-
-      shell->hsbdata->upper = MAX (value + shell->disp_width,
-                                   sw);
     }
   else
     {
-      shell->hsbdata->lower = MIN (value,
+      lower = MIN (value, -(shell->disp_width - sw) / 2);
-                                   -(shell->disp_width - sw) / 2);
+      upper = MAX (value + shell->disp_width,
-
+                   sw + (shell->disp_width - sw) / 2);
-      shell->hsbdata->upper = MAX (value + shell->disp_width,
-                                   sw + (shell->disp_width - sw) / 2);
     }
 
-  shell->hsbdata->step_increment = MAX (shell->scale_x, MINIMUM_STEP_AMOUNT);
+  g_object_set (shell->hsbdata,
+                "lower",          lower,
+                "upper",          upper,
+                "step-increment", (gdouble) MAX (shell->scale_x,
+                                                 MINIMUM_STEP_AMOUNT),
+                NULL);
 }
 
 /**
@@ -588,7 +590,9 @@ void
 gimp_display_shell_scroll_setup_vscrollbar (GimpDisplayShell *shell,
                                             gdouble           value)
 {
-  gint sh;
+  gint    sh;
+  gdouble lower;
+  gdouble upper;
 
   g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
 
@@ -600,20 +604,20 @@ gimp_display_shell_scroll_setup_vscrollbar (GimpDisplayShell *shell,
 
   if (shell->disp_height < sh)
     {
-      shell->vsbdata->lower = MIN (value,
+      lower = MIN (value, 0);
-                                   0);
+      upper = MAX (value + shell->disp_height, sh);
-
-      shell->vsbdata->upper = MAX (value + shell->disp_height,
-                                   sh);
     }
   else
     {
-      shell->vsbdata->lower = MIN (value,
+      lower = MIN (value, -(shell->disp_height - sh) / 2);
-                                   -(shell->disp_height - sh) / 2);
+      upper = MAX (value + shell->disp_height,
-
+                   sh + (shell->disp_height - sh) / 2);
-      shell->vsbdata->upper = MAX (value + shell->disp_height,
-                                   sh + (shell->disp_height - sh) / 2);
     }
 
-  shell->vsbdata->step_increment = MAX (shell->scale_y, MINIMUM_STEP_AMOUNT);
+  g_object_set (shell->vsbdata,
+                "lower",          lower,
+                "upper",          upper,
+                "step-increment", (gdouble) MAX (shell->scale_y,
+                                                 MINIMUM_STEP_AMOUNT),
+                NULL);
 }