gridview: Make focus movement section-aware

Make up-down focus movements preserve the column
across section boundaries.

This code does the simplest thing that can work.
If this turns out to be a problem, it can be
made smarter.

Note that the code currently prefers to jump over
short sections to preserve the column. If there
are many short sections, this not be desired,
and we may want to make it tweakable.

gtk/gtkgridview.c

@@ -619,26 +619,76 @@ gtk_grid_view_get_items_in_rect (GtkListBase        *base,
   return result;
 }
 
+static unsigned int
+find_previous_item_in_column (GtkGridView  *self,
+                              unsigned int  position)
+{
+  unsigned int col, pos;
+
+  if (position == 0)
+    return position;
+
+  col = gtk_grid_view_get_column_for_position (self->item_manager,
+                                               self->n_columns,
+                                               position);
+
+  pos = position;
+  do
+    {
+      pos--;
+
+      if (col == gtk_grid_view_get_column_for_position (self->item_manager,
+                                                        self->n_columns,
+                                                        pos))
+        return pos;
+
+    } while (pos > 0);
+
+  return position;
+}
+
+static unsigned int
+find_next_item_in_column (GtkGridView  *self,
+                          unsigned int  position)
+{
+  unsigned int col;
+  unsigned int n_items;
+
+  n_items = g_list_model_get_n_items (G_LIST_MODEL (gtk_list_item_manager_get_model (self->item_manager)));
+
+  col = gtk_grid_view_get_column_for_position (self->item_manager,
+                                               self->n_columns,
+                                               position);
+
+  for (unsigned int p = position + 1; p < n_items; p++)
+    {
+      if (col == gtk_grid_view_get_column_for_position (self->item_manager,
+                                                        self->n_columns,
+                                                        p))
+        return p;
+    }
+
+  return position;
+}
+
 static guint
 gtk_grid_view_move_focus_along (GtkListBase *base,
                                 guint        pos,
                                 int          steps)
 {
   GtkGridView *self = GTK_GRID_VIEW (base);
+  unsigned int prev_pos = pos;
 
-  steps *= self->n_columns;
+  for (unsigned int i = 0; i < abs (steps); i++)
+    {
+      if (steps < 0)
+        pos = find_previous_item_in_column (self, pos);
+      else
+        pos = find_next_item_in_column (self, pos);
+    }
 
-  if (steps < 0)
-    {
-      if (pos >= self->n_columns)
-        pos -= MIN (pos, -steps);
-    }
-  else
-    {
-      guint n_items = gtk_list_base_get_n_items (base);
-      if (n_items / self->n_columns > pos / self->n_columns)
-        pos += MIN (n_items - pos - 1, steps);
-    }
+  if (prev_pos == pos)
+    gtk_widget_keynav_failed (GTK_WIDGET (self), steps < 0 ? GTK_DIR_UP : GTK_DIR_DOWN);
 
   return pos;
 }
@@ -648,14 +698,19 @@ gtk_grid_view_move_focus_across (GtkListBase *base,
                                  guint        pos,
                                  int          steps)
 {
+  unsigned int prev_pos = pos;
+
   if (steps < 0)
-    return pos - MIN (pos, -steps);
+    pos = pos - MIN (pos, -steps);
   else
     {
       guint n_items = gtk_list_base_get_n_items (base);
       pos += MIN (n_items - pos - 1, steps);
     }
 
+  if (prev_pos == pos)
+    gtk_widget_keynav_failed (GTK_WIDGET (base), steps < 0 ? GTK_DIR_LEFT : GTK_DIR_RIGHT);
+
   return pos;
 }