magnifier: Correct position of area

The widget paintable uses the widgets bounds
as intrinsic size, so we need to offset from
that to the allocation, which is what the
coordinates are relative to.

gtk/gtkmagnifier.c

@@ -20,6 +20,7 @@
 #include "gtkmagnifierprivate.h"
 #include "gtkwidgetprivate.h"
 #include "gtksnapshot.h"
+#include "gtkcssboxesprivate.h"
 
 enum {
   PROP_INSPECTED = 1,
@@ -105,10 +106,23 @@ gtk_magnifier_snapshot (GtkWidget   *widget,
 {
   GtkMagnifier *magnifier = GTK_MAGNIFIER (widget);
   double width, height, paintable_width, paintable_height;
+  GtkWidget *inspected;
+  GtkCssBoxes boxes;
+  const graphene_rect_t *content_rect;
+  const graphene_rect_t *border_rect;
+  graphene_point_t offset;
 
-  if (gtk_widget_paintable_get_widget (GTK_WIDGET_PAINTABLE (magnifier->paintable)) == NULL)
+  inspected = gtk_widget_paintable_get_widget (GTK_WIDGET_PAINTABLE (magnifier->paintable));
+  if (inspected == NULL)
     return;
 
+  gtk_css_boxes_init (&boxes, inspected);
+  content_rect = gtk_css_boxes_get_content_rect (&boxes);
+  border_rect = gtk_css_boxes_get_border_rect (&boxes);
+
+  offset.x = content_rect->origin.x - border_rect->origin.x;
+  offset.y = content_rect->origin.y - border_rect->origin.y;
+
   width = gtk_widget_get_width (widget);
   height = gtk_widget_get_height (widget);
   paintable_width = gdk_paintable_get_intrinsic_width (magnifier->paintable);
@@ -121,8 +135,8 @@ gtk_magnifier_snapshot (GtkWidget   *widget,
     gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (width / 2, height / 2));
   gtk_snapshot_scale (snapshot, magnifier->magnification, magnifier->magnification);
   gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (
-                          - CLAMP (magnifier->x, 0, paintable_width),
-                          - CLAMP (magnifier->y, 0, paintable_height)));
+                          - CLAMP (magnifier->x + offset.x, 0, paintable_width),
+                          - CLAMP (magnifier->y + offset.y, 0, paintable_height)));
 
   gdk_paintable_snapshot (magnifier->paintable, snapshot, paintable_width, paintable_height);
   gtk_snapshot_restore (snapshot);