Support moving dock clients to another output.

This fixes #1883 where a race condition between i3 and i3bar caused two i3bar clients to be put onto the same output.

src/handlers.c

@@ -386,15 +386,35 @@ static void handle_configure_request(xcb_configure_request_event_t *event) {
         return;
     }
 
-    /* Dock windows can be reconfigured in their height */
+    /* Dock windows can be reconfigured in their height and moved to another output. */
     if (con->parent && con->parent->type == CT_DOCKAREA) {
-        DLOG("Dock window, only height reconfiguration allowed\n");
+        DLOG("Reconfiguring dock window (con = %p).\n", con);
         if (event->value_mask & XCB_CONFIG_WINDOW_HEIGHT) {
-            DLOG("Height given, changing\n");
+            DLOG("Dock client wants to change height to %d, we can do that.\n", event->height);
 
             con->geometry.height = event->height;
             tree_render();
         }
+
+        if (event->value_mask & XCB_CONFIG_WINDOW_X || event->value_mask & XCB_CONFIG_WINDOW_Y) {
+            int16_t x = event->value_mask & XCB_CONFIG_WINDOW_X ? event->x : (int16_t)con->geometry.x;
+            int16_t y = event->value_mask & XCB_CONFIG_WINDOW_Y ? event->y : (int16_t)con->geometry.y;
+
+            Con *current_output = con_get_output(con);
+            Output *target = get_output_containing(x, y);
+            if (target != NULL && current_output != target->con) {
+                DLOG("Dock client is requested to be moved to output %s, moving it there.\n", target->name);
+                Match *match;
+                Con *nc = con_for_window(target->con, con->window, &match);
+                DLOG("Dock client will be moved to container %p.\n", nc);
+                con_detach(con);
+                con_attach(con, nc, false);
+
+                tree_render();
+            } else {
+                DLOG("Dock client will not be moved, we only support moving it to another output.\n");
+            }
+        }
     }
 
     fake_absolute_configure_notify(con);

testcases/t/526-reconfigure-dock.t

@@ -0,0 +1,57 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://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 reconfiguration of dock clients.
+# Ticket: #1883
+use i3test i3_autostart => 0;
+
+my ($config, $pid, $window, $rect);
+my (@docks);
+
+###############################################################################
+# 1: Given two screens A and B and a dock client on screen A, when the dock
+#    client is reconfigured to be positioned on screen B, then the client is
+#    moved to the correct position.
+###############################################################################
+
+$config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+fake-outputs 1024x768+0+0,1024x768+1024+0
+EOT
+$pid = launch_with_config($config);
+
+$window = open_window({
+        window_type => $x->atom(name => '_NET_WM_WINDOW_TYPE_DOCK')
+    });
+
+$rect = $window->rect;
+is($rect->x, 0, 'sanity check: dock client is on the left screen');
+
+$window->rect(X11::XCB::Rect->new(x => 1024, y => 0, width => 1024, height => 30));
+sync_with_i3;
+
+@docks = get_dock_clients;
+is(@docks, 1, 'there is still exactly one dock');
+
+is($docks[0]->{rect}->{x}, 1024, 'dock client has moved to the other screen');
+
+exit_gracefully($pid);
+
+###############################################################################
+
+done_testing;