Set EWMH active window to None when none has focus

_NET_ACTIVE_WINDOW:

> The window ID of the currently active window or None if no window has
> the focus.

This fixes a bug that would not update _NET_ACTIVE_WINDOW when focus
changed to an i3 container without a window such as a branch or
workspace content container.

src/x.c

@@ -1013,6 +1013,8 @@ void x_push_changes(Con *con) {
                      to_focus, focused, focused->name);
                 send_take_focus(to_focus, last_timestamp);
 
+                ewmh_update_active_window((con_has_managed_window(focused) ? focused->window->id : XCB_WINDOW_NONE));
+
                 if (to_focus != last_focused && is_con_attached(focused))
                    ipc_send_window_event("focus", focused);
             } else {
@@ -1030,7 +1032,7 @@ void x_push_changes(Con *con) {
                     xcb_change_window_attributes(conn, focused->window->id, XCB_CW_EVENT_MASK, values);
                 }
 
-                ewmh_update_active_window(to_focus);
+                ewmh_update_active_window((con_has_managed_window(focused) ? focused->window->id : XCB_WINDOW_NONE));
 
                 if (to_focus != XCB_NONE && to_focus != last_focused && focused->window != NULL && is_con_attached(focused))
                    ipc_send_window_event("focus", focused);
@@ -1043,6 +1045,7 @@ void x_push_changes(Con *con) {
     if (focused_id == XCB_NONE) {
         DLOG("Still no window focused, better set focus to the root window\n");
         xcb_set_input_focus(conn, XCB_INPUT_FOCUS_POINTER_ROOT, root, XCB_CURRENT_TIME);
+        ewmh_update_active_window(XCB_WINDOW_NONE);
         focused_id = root;
     }
 

testcases/t/195-net-active-window.t

@@ -15,7 +15,8 @@
 #   (unless you are already familiar with Perl)
 #
 # Verifies that the _NET_ACTIVE_WINDOW message only changes focus when the
-# window is on a visible workspace.
+# window is on a visible workspace and that focus changes properly update this
+# property on the root window.
 # ticket #774, bug still present in commit 1e49f1b08a3035c1f238fcd6615e332216ab582e
 # ticket #1136, bug still present in commit fd07f989fdf441ef335245dd3436a70ff60e8896
 use i3test;
@@ -40,6 +41,23 @@ sub send_net_active_window {
     $x->send_event(0, $x->get_root_window(), X11::XCB::EVENT_MASK_SUBSTRUCTURE_REDIRECT, $msg);
 }
 
+sub get_net_active_window {
+    my $cookie = $x->get_property(
+        0,
+        $x->get_root_window(),
+        $x->atom(name => '_NET_ACTIVE_WINDOW')->id,
+        $x->atom(name => 'WINDOW')->id,
+        0,
+        4096,
+    );
+    my $reply = $x->get_property_reply($cookie->{sequence});
+    my $len = $reply->{length};
+
+    return -1 if $len == 0;
+    return unpack("L", $reply->{value});
+
+}
+
 my $ws1 = fresh_workspace;
 my $win1 = open_window;
 my $win2 = open_window;
@@ -113,4 +131,34 @@ send_net_active_window($scratch->id);
 
 is($x->input_focus, $win3->id, 'window 3 still focused');
 
+################################################################################
+# Verify that the _NET_ACTIVE_WINDOW property is updated on the root window
+# correctly.
+################################################################################
+
+fresh_workspace;
+
+sync_with_i3;
+
+is(get_net_active_window(), 0, 'workspace content focus is indicated by the root property as "None" window');
+
+my $win4 = open_window;
+
+cmd '[id="' . $win4->id . '"] focus';
+
+sync_with_i3;
+
+is(get_net_active_window(), $win4->id, 'window 4 is indicated as focused by the root property');
+
+# make a branch
+open_window;
+open_window;
+cmd 'split h';
+open_window;
+cmd 'focus parent';
+
+sync_with_i3;
+
+is(get_net_active_window(), 0, 'branch focus is indicated by the root property as "None" window');
+
 done_testing;