Don't always clobber the viewport Y when changing canvas grid.

src/canvas.C

@@ -109,7 +109,7 @@ Canvas::grid ( Grid *g )
 
     resize_grid();
 
-    changed_mapping();
+    update_mapping();
 
     m.shape = m.grid->draw_shape();
 
@@ -152,16 +152,14 @@ Canvas::_update_row_mapping ( void )
     m.vp->h = min( m.vp->h, m.maxh );
 }
 
-/** change grid mapping */
+/** update everything about mapping, leaving the viewport alone */
 void
-Canvas::changed_mapping ( void )
+Canvas::update_mapping ( void )
 {
     _update_row_mapping();
 
     m.mapping_drawn = false;
 
-    m.vp->y = (m.maxh / 2) - (m.vp->h / 2);
-
     resize();
 
     int old_margin = m.margin_left;
@@ -181,6 +179,18 @@ Canvas::changed_mapping ( void )
         signal_draw();
 }
 
+/** change grid mapping */
+void
+Canvas::changed_mapping ( void )
+{
+    update_mapping();
+
+    m.vp->h = min( m.vp->h, m.maxh );
+
+    if ( m.vp->y + m.vp->h > m.maxh )
+        m.vp->y = (m.maxh / 2) - (m.vp->h / 2);
+}
+
 Grid *
 Canvas::grid ( void )
 {

src/canvas.H

@@ -109,6 +109,8 @@ class Canvas : public trackable
     bool viewable_x ( int x );
     void draw_line ( int x, int flags );
 
+    void update_mapping ( void );
+
 public:
 
     enum { OFF, ON, TOGGLE };