Avoid creating redundant containers when switching between tabbed/stacked and split layouts (#5469)

Fixes #3001

release-notes/changes/3-redundant-containers

@@ -0,0 +1 @@
+avoid creating redundant containers when switching between tabbed/stacked and split layouts

src/con.c

@@ -2063,6 +2063,31 @@ void con_set_layout(Con *con, layout_t layout) {
         }
     }
 
+    if (con->type != CT_WORKSPACE && con->parent->type != CT_WORKSPACE &&
+        con_num_children(con) == 1 && con_num_children(con->parent) == 1) {
+        /* Special case: Avoid creating redundant containers (#3001):
+         * split h / v (tree_split()) will avoid creating new containers when
+         * the target container is already a single child in L_SPLITH /
+         * L_SPLITV. However, if the layout is tabbed / stacked, a new split is
+         * created. This means, however, that when the user continuously
+         * switches between split h/v and tabbed / stacked, an endless series
+         * of 1-child containers will be created. Since a single level of split
+         * containers on top of tabbed / stacked containers are useful, we want
+         * to avoid this situation here.
+         * Example of past behaviour: S[V[w]] -> S[S[w]] -> S[S[V[w]]] -> …
+         * Example of desired behaviour: S[V[w]] -> S[w] -> S[v[w]] -> …
+         * Therefore, when both the current & parent containers have a single
+         * child, we just close the redundant middle container and proceed with
+         * the parent. */
+        Con *parent = con->parent;
+        Con *child = TAILQ_FIRST(&(con->nodes_head));
+        con_detach(child);
+        con_attach(child, parent, true);
+        parent->last_split_layout = con->last_split_layout;
+        tree_close_internal(con, DONT_KILL_WINDOW, true);
+        con = parent;
+    }
+
     if (layout == L_DEFAULT) {
         /* Special case: the layout formerly known as "default" (in combination
          * with an orientation). Since we switched to splith/splitv layouts,

testcases/t/550-split-redundant-containers.t

@@ -0,0 +1,82 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test that splitting and stacked/tabbed layouts do not create redundant
+# containers.
+# Ticket: #3001
+# Bug still in: 4.22-24-ga5da4d54
+use i3test;
+
+cmp_tree(
+    msg => 'toggling between split h/v',
+    layout_before => 'H[a*]',
+    layout_after => 'V[a*]',
+    cb => sub {
+        cmd 'split v, split h, split v';
+    });
+cmp_tree(
+    msg => 'toggling between tabbed/stacked',
+    layout_before => 'H[a*]',
+    layout_after => 'S[a*]',
+    cb => sub {
+        cmd 'layout tabbed, layout stacked';
+    });
+cmp_tree(
+    msg => 'split h to v and then tabbed',
+    layout_before => 'H[a*]',
+    layout_after => 'T[a*]',
+    cb => sub {
+        cmd 'split v, layout tabbed';
+    });
+cmp_tree(
+    msg => 'repeat tabbed layout',
+    layout_before => 'H[a*]',
+    layout_after => 'T[a*]',
+    cb => sub {
+        cmd 'layout tabbed' for 1..5;
+    });
+cmp_tree(
+    msg => 'split v inside tabbed',
+    layout_before => 'H[a*]',
+    layout_after => 'T[V[a*]]',
+    cb => sub {
+        cmd 'layout tabbed, split v';
+    });
+cmp_tree(
+    msg => 'split v inside tabbed and back to just tabbed',
+    layout_before => 'H[a*]',
+    layout_after => 'T[a*]',
+    cb => sub {
+        cmd 'layout tabbed, split v, layout tabbed';
+    });
+cmp_tree(
+    msg => 'toggle split v inside tabbed',
+    layout_before => 'H[a*]',
+    layout_after => 'T[V[a*]]',
+    cb => sub {
+        cmd 'layout tabbed, split v, layout tabbed, split v';
+    });
+cmp_tree(
+    msg => 'tabbed with 2 nodes inside other tabbed',
+    layout_before => 'T[a*]',
+    layout_after => 'T[T[a b*]]',
+    cb => sub {
+        cmd 'split v';
+        open_window(wm_class => "b", name => "b");
+        cmd 'layout tabbed';
+    });
+
+done_testing;