Window decoration scrolling: don't focus sibling

The current behaviour is buggy in the following layout:
T [ A* V [ B C ] ], where the focus stack in V is B > C.
When the user scrolls down, focus correctly moves to B but if the user
scrolls down again the whole vertical container is focused.

This happens because 'bool scroll_next_possible' is false but
con_activate is called on the tabbed container's sibling - the vertical
container.

src/click.c

@@ -234,14 +234,10 @@ static int route_click(Con *con, xcb_button_press_event_t *event, const bool mod
          event->detail == XCB_BUTTON_SCROLL_RIGHT)) {
         DLOG("Scrolling on a window decoration\n");
         orientation_t orientation = (con->parent->layout == L_STACKED ? VERT : HORIZ);
-        /* Focus the currently focused container on the same level that the
+        /* Use the focused child of the tabbed / stacked container, not the
-         * user scrolled on. e.g. the tabbed decoration contains
+         * container the user scrolled on. */
-         * "urxvt | i3: V[xterm geeqie] | firefox",
-         * focus is on the xterm, but the user scrolled on urxvt.
-         * The splitv container will be focused. */
         Con *focused = con->parent;
         focused = TAILQ_FIRST(&(focused->focus_head));
-        con_activate(focused);
         /* To prevent scrolling from going outside the container (see ticket
          * #557), we first check if scrolling is possible at all. */
         bool scroll_prev_possible = (TAILQ_PREV(focused, nodes_head, nodes) != NULL);

testcases/t/297-scroll-tabbed.t

@@ -0,0 +1,79 @@
+#!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)
+#
+# Tests if scrolling the tab bar on a tabbed container works and verifies that
+# only one window is focused as a result.
+# Ticket: #3215 (PR)
+# Bug still in: 4.15-92-g666aa9e0
+use i3test;
+use i3test::XTEST;
+
+sub scroll_down {
+    # button5 = scroll down
+    xtest_button_press(5, 3, 3);
+    xtest_button_release(5, 3, 3);
+    xtest_sync_with_i3;
+}
+
+sub scroll_up {
+    # button4 = scroll up
+    xtest_button_press(4, 3, 3);
+    xtest_button_release(4, 3, 3);
+    xtest_sync_with_i3;
+}
+
+# Decoration of top left window.
+$x->root->warp_pointer(3, 3);
+
+# H [ T [ H [ A B ] C D V [ E F ] ] G ]
+# Inner horizontal split.
+open_window;
+cmd 'layout tabbed';
+cmd 'splith';
+my $first = open_window;
+cmd 'focus parent';
+# Simple tabs.
+open_window;
+my $second_last = open_window;
+# V-Split container
+open_window;
+cmd 'splitv';
+my $last = open_window;
+# Second child of the outer horizontal split, next to the tabbed one.
+open_window;
+cmd 'move right, move right';
+
+cmd '[id=' . $first->id . '] focus';
+
+# Scroll from first to last.
+scroll_down;
+scroll_down;
+is($x->input_focus, $second_last->id, 'Sanity check: scrolling');
+scroll_down;
+is($x->input_focus, $last->id, 'Last window focused through scrolling');
+scroll_down;
+is($x->input_focus, $last->id, 'Scrolling again doesn\'t leave the tabbed container and doesn\'t focus the whole sibling');
+
+# Scroll from last to first.
+scroll_up;
+is($x->input_focus, $second_last->id, 'Scrolling up works');
+scroll_up;
+scroll_up;
+is($x->input_focus, $first->id, 'First window focused through scrolling');
+scroll_up;
+is($x->input_focus, $first->id, 'Scrolling again doesn\'t focus the whole sibling');
+
+done_testing;