Treat stacking containers as if they are in vertical orientation, add testcase

src/con.c

@@ -376,3 +376,17 @@ void con_move_to_workspace(Con *con, Con *workspace) {
     con_detach(con);
     con_attach(con, next);
 }
+
+/*
+ * Returns the orientation of the given container (for stacked containers,
+ * vertical orientation is used regardless of the actual orientation of the
+ * container).
+ *
+ */
+int con_orientation(Con *con) {
+    /* stacking containers behave like they are in vertical orientation */
+    if (con->layout == L_STACKED)
+        return VERT;
+
+    return con->orientation;
+}

src/tree.c

@@ -332,7 +332,7 @@ void tree_render() {
 void tree_next(char way, orientation_t orientation) {
     /* 1: get the first parent with the same orientation */
     Con *parent = focused->parent;
-    while (parent->orientation != orientation) {
+    while (con_orientation(parent) != orientation) {
         LOG("need to go one level further up\n");
         /* if the current parent is an output, we are at a workspace
          * and the orientation still does not match */
@@ -377,7 +377,7 @@ void tree_move(char way, orientation_t orientation) {
     if (focused->type == CT_WORKSPACE)
         return;
     bool level_changed = false;
-    while (parent->orientation != orientation) {
+    while (con_orientation(parent) != orientation) {
         LOG("need to go one level further up\n");
         /* if the current parent is an output, we are at a workspace
          * and the orientation still does not match */