Correct insert_con_into's focus handling

Change from always putting con on the head of the new parent. Important
for moving unfocused containers.

src/move.c

@@ -12,6 +12,54 @@
 typedef enum { BEFORE,
                AFTER } position_t;
 
+/*
+ * Returns the lowest container in the tree that has both a and b as descendants.
+ *
+ */
+static Con *lowest_common_ancestor(Con *a, Con *b) {
+    Con *parent_a = a;
+    while (parent_a) {
+        Con *parent_b = b;
+        while (parent_b) {
+            if (parent_a == parent_b) {
+                return parent_a;
+            }
+            parent_b = parent_b->parent;
+        }
+        parent_a = parent_a->parent;
+    }
+    assert(false);
+}
+
+/*
+ * Returns the direct child of ancestor that contains con.
+ *
+ */
+static Con *child_containing_con_recursively(Con *ancestor, Con *con) {
+    Con *child = con;
+    while (child && child->parent != ancestor) {
+        child = child->parent;
+        assert(child->parent);
+    }
+    return child;
+}
+
+/*
+ * Returns true if the given container is the focused descendant of ancestor, recursively.
+ *
+ */
+static bool is_focused_descendant(Con *con, Con *ancestor) {
+    Con *current = con;
+    while (current != ancestor) {
+        if (TAILQ_FIRST(&(current->parent->focus_head)) != current) {
+            return false;
+        }
+        current = current->parent;
+        assert(current->parent);
+    }
+    return true;
+}
+
 /*
  * This function detaches 'con' from its parent and inserts it either before or
  * after 'target'.
@@ -24,6 +72,52 @@ static void insert_con_into(Con *con, Con *target, position_t position) {
      * afterwards which might then close the con if it is empty. */
     Con *old_parent = con->parent;
 
+    /* We compare the focus order of the children of the lowest common ancestor. If con or
+     * its ancestor is before target's ancestor then con should be placed before the target
+     * in the focus stack. */
+    Con *lca = lowest_common_ancestor(con, parent);
+    if (lca == con) {
+        ELOG("Container is being inserted into one of its descendants.\n");
+        return;
+    }
+
+    Con *con_ancestor = child_containing_con_recursively(lca, con);
+    Con *target_ancestor = child_containing_con_recursively(lca, target);
+    bool moves_focus_from_ancestor = is_focused_descendant(con, con_ancestor);
+    bool focus_before;
+
+    /* Determine if con is going to be placed before or after target in the parent's focus stack. */
+    if (con_ancestor == target_ancestor) {
+        /* Happens when the target is con's old parent. Eg with layout V [ A H [ B C ] ],
+         * if we move C up. Target will be H. */
+        focus_before = moves_focus_from_ancestor;
+    } else {
+        /* Look at the focus stack order of the children of the lowest common ancestor. */
+        Con *current;
+        TAILQ_FOREACH(current, &(lca->focus_head), focused) {
+            if (current == con_ancestor || current == target_ancestor) {
+                break;
+            }
+        }
+        focus_before = (current == con_ancestor);
+    }
+
+    /* If con is the focused container in our old ancestor we place the new ancestor
+     * before the old ancestor in the focus stack. Example:
+     * Consider the layout [ H [ V1 [ A* B ] V2 [ C ] ] ] where A is focused. We move to
+     * a second workspace and from there we move A to the right and switch back to the
+     * original workspace. Without the change focus would move to B instead of staying
+     * with A. */
+    if (moves_focus_from_ancestor && focus_before) {
+        Con *place = TAILQ_PREV(con_ancestor, focus_head, focused);
+        TAILQ_REMOVE(&(lca->focus_head), target_ancestor, focused);
+        if (place) {
+            TAILQ_INSERT_AFTER(&(lca->focus_head), place, target_ancestor, focused);
+        } else {
+            TAILQ_INSERT_HEAD(&(lca->focus_head), target_ancestor, focused);
+        }
+    }
+
     con_detach(con);
     con_fix_percent(con->parent);
 
@@ -46,12 +140,28 @@ static void insert_con_into(Con *con, Con *target, position_t position) {
 
     con->parent = parent;
 
+    if (parent == lca) {
+        if (focus_before) {
+            /* Example layout: H [ A B* ], we move A up/down. 'target' will be H. */
+            TAILQ_INSERT_BEFORE(target, con, focused);
+        } else {
+            /* Example layout: H [ A B* ], we move A up/down. 'target' will be H. */
+            TAILQ_INSERT_AFTER(&(parent->focus_head), target, con, focused);
+        }
+    } else {
+        if (focus_before) {
+            /* Example layout: V [ H [ A B ] C* ], we move C up. 'target' will be A. */
+            TAILQ_INSERT_HEAD(&(parent->focus_head), con, focused);
+        } else {
+            /* Example layout: V [ H [ A* B ] C ], we move C up. 'target' will be A. */
+            TAILQ_INSERT_TAIL(&(parent->focus_head), con, focused);
+        }
+    }
+
     if (position == BEFORE) {
         TAILQ_INSERT_BEFORE(target, con, nodes);
-        TAILQ_INSERT_HEAD(&(parent->focus_head), con, focused);
     } else if (position == AFTER) {
         TAILQ_INSERT_AFTER(&(parent->nodes_head), target, con, nodes);
-        TAILQ_INSERT_HEAD(&(parent->focus_head), con, focused);
     }
 
     /* Pretend the con was just opened with regards to size percent values.