Support _NET_WM_STATE_FOCUSED

_NET_WM_STATE_FOCUSED is set on _NET_WM_STATE to indicate that the
window is focused. It must be set when the window is newly focused and
removed once the window no longer has focus.

> _NET_WM_STATE_FOCUSED indicates whether the window's decorations are
> drawn in an active state. Clients MUST regard it as a read-only hint.
> It cannot be set at map time or changed via a _NET_WM_STATE client
> message.

For example, this is used by GTK applications to show the decoration in
an active or inactive state. This change can be tested by opening a GTK
application (like evince), focusing the window and unfocusing the
window, and observing a change in the window decorations.

Fixes #2273

testcases/lib/i3test.pm.in

@@ -51,6 +51,7 @@ our @EXPORT = qw(
     kill_all_windows
     events_for
     listen_for_binding
+    is_net_wm_state_focused
 );
 
 =head1 NAME
@@ -1026,6 +1027,40 @@ sub listen_for_binding {
     return $command;
 }
 
+=head2 is_net_wm_state_focused
+
+Returns true if the given window has the _NET_WM_STATE_FOCUSED atom.
+
+    ok(is_net_wm_state_focused($window), '_NET_WM_STATE_FOCUSED set');
+
+=cut
+sub is_net_wm_state_focused {
+    my ($window) = @_;
+
+    sync_with_i3;
+    my $atom = $x->atom(name => '_NET_WM_STATE_FOCUSED');
+    my $cookie = $x->get_property(
+        0,
+        $window->{id},
+        $x->atom(name => '_NET_WM_STATE')->id,
+        GET_PROPERTY_TYPE_ANY,
+        0,
+        4096
+    );
+
+    my $reply = $x->get_property_reply($cookie->{sequence});
+    my $len = $reply->{length};
+    return 0 if $len == 0;
+
+    my @atoms = unpack("L$len", $reply->{value});
+    for (my $i = 0; $i < $len; $i++) {
+        return 1 if $atoms[$i] == $atom->id;
+    }
+
+    return 0;
+}
+
+
 =head1 AUTHOR
 
 Michael Stapelberg <michael@i3wm.org>