wl: Prefer SSD via zxdg_decoration_manager_v1

Ask compositors that support the zxdg_decoration_manager_v1_interface
protocol to provide server-side decorations (SSD). This way it is more
likely that the Cog surfaces will have window decorations. Note that
compositors will still hide the decorations of fullscreen surfaces,
but maximized ones will keep them, and this is the expected behaviour.
Tested on Sway and Labwc, and should work in most (if not all) wlroots
based compositors.

platform/wayland/cog-platform-wl.c

@@ -212,6 +212,10 @@ registry_on_global(void *data, struct wl_registry *registry, uint32_t name, cons
     } else if (strcmp(interface, weston_content_protection_interface.name) == 0) {
         display->protection = wl_registry_bind(registry, name, &weston_content_protection_interface, 1);
 #endif /* COG_ENABLE_WESTON_CONTENT_PROTECTION */
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+    } else if (strcmp(interface, zxdg_decoration_manager_v1_interface.name) == 0) {
+        display->xdg_decoration = wl_registry_bind(registry, name, &zxdg_decoration_manager_v1_interface, 1);
+#endif /* COG_HAVE_XDG_DECORATION_UNSTABLE_V1 */
     } else if (strcmp(interface, wl_output_interface.name) == 0) {
         /* Version 2 introduced the wl_output_listener::scale. */
         CogWlOutput *item = g_new0(CogWlOutput, 1);
@@ -1181,6 +1185,10 @@ init_wayland(CogWlPlatform *platform, GError **error)
 static void
 clear_wayland(CogWlPlatform *platform)
 {
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+    g_clear_pointer(&platform->display->xdg_decoration, zxdg_decoration_manager_v1_destroy);
+#endif /* COG_HAVE_XDG_DECORATION_UNSTABLE_V1 */
+
     cog_wl_display_destroy(platform->display);
 }
 

platform/wayland/cog-utils-wl.h

@@ -15,6 +15,10 @@
 #    include "cog-xdp-parent-wl.h"
 #endif
 
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+#    include "xdg-decoration-unstable-v1-client.h"
+#endif
+
 #include <wayland-server.h>
 #include <wayland-util.h>
 #include <xkbcommon/xkbcommon.h>
@@ -116,6 +120,10 @@ struct _CogWlWindow {
     struct xdg_toplevel     *xdg_toplevel;
     struct wl_shell_surface *shell_surface;
 
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+    struct zxdg_toplevel_decoration_v1 *xdg_decoration;
+#endif /* COG_HAVE_XDG_DECORATION_UNSTABLE_V1 */
+
 #if COG_HAVE_LIBPORTAL
     struct xdp_parent_wl_data xdp_parent_wl_data;
 #endif /* COG_HAVE_LIBPORTAL */
@@ -231,6 +239,10 @@ struct _CogWlDisplay {
     struct zwp_fullscreen_shell_v1 *fshell;
     struct wl_shell                *shell;
 
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+    struct zxdg_decoration_manager_v1 *xdg_decoration;
+#endif
+
     CogWlSeat     *seat_default;
     struct wl_list seats; /* wl_list<CogWlSeat> */
 

platform/wayland/cog-viewport-wl.c

@@ -16,6 +16,10 @@
 #include "fullscreen-shell-unstable-v1-client.h"
 #include "xdg-shell-client.h"
 
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+#    include "xdg-decoration-unstable-v1-client.h"
+#endif
+
 G_DEFINE_DYNAMIC_TYPE(CogWlViewport, cog_wl_viewport, COG_TYPE_VIEWPORT)
 
 static void cog_wl_viewport_dispose(GObject *);
@@ -41,6 +45,10 @@ destroy_video_surface(gpointer data)
 static void
 destroy_window(CogWlViewport *viewport)
 {
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+    g_clear_pointer(&viewport->window.xdg_decoration, zxdg_toplevel_decoration_v1_destroy);
+#endif
+
     g_clear_pointer(&viewport->window.xdg_toplevel, xdg_toplevel_destroy);
     g_clear_pointer(&viewport->window.xdg_surface, xdg_surface_destroy);
     g_clear_pointer(&viewport->window.shell_surface, wl_shell_surface_destroy);
@@ -265,6 +273,19 @@ cog_wl_viewport_create_window(CogWlViewport *viewport, GError **error)
         viewport->window.xdg_toplevel = xdg_surface_get_toplevel(viewport->window.xdg_surface);
         g_assert(viewport->window.xdg_toplevel);
 
+#if COG_HAVE_XDG_DECORATION_UNSTABLE_V1
+        /*
+         * Ask the compositor for server-side decorations. The compositor may
+         * still prefer client-side decorations, but do not attach a listener
+         * because knowing the compositor's preference is pointless because
+         * there is no support for painting client-side decorations.
+         */
+        viewport->window.xdg_decoration =
+            zxdg_decoration_manager_v1_get_toplevel_decoration(display->xdg_decoration, viewport->window.xdg_toplevel);
+        zxdg_toplevel_decoration_v1_set_mode(viewport->window.xdg_decoration,
+                                             ZXDG_TOPLEVEL_DECORATION_V1_MODE_SERVER_SIDE);
+#endif /* COG_HAVE_XDG_DECORATION_UNSTABLE_V1 */
+
         xdg_toplevel_add_listener(viewport->window.xdg_toplevel, &xdg_toplevel_listener, viewport);
         xdg_toplevel_set_title(viewport->window.xdg_toplevel, COG_DEFAULT_APPNAME);
 

platform/wayland/meson.build

@@ -38,6 +38,7 @@ wayland_platform_protocols = {
         ['text-input', 1],
         ['text-input', 3],
         ['xdg-foreign', 2],
+        ['xdg-decoration', 1, 'optional'],
     ],
     'weston': wayland_platform_weston_protocols,
 }
@@ -78,22 +79,34 @@ fs = import('fs')
 wayland_platform_sources = []
 foreach kind, proto_list : wayland_platform_protocols
     foreach item : proto_list
+        proto_optional = false
         if kind == 'stable'
             proto_name = item
             proto_dir = join_paths(wayland_protocols_path, 'stable', item)
         elif kind == 'unstable' or kind == 'staging'
             proto_name = '@0@-@1@-v@2@'.format(item[0], kind, item[1])
             proto_dir = join_paths(wayland_protocols_path, 'unstable', item[0])
+            proto_optional = item.length() == 3 and item[2] == 'optional'
         elif kind == 'weston'
             proto_name = item
             proto_dir = weston_protocols_path
         else
             error('Unknown Wayland protocol type: @0@'.format(kind))
         endif
+        proto_macro = proto_name.underscorify().to_upper()
 
         xml_path = join_paths(proto_dir, '@0@.xml'.format(proto_name))
         if not fs.is_file(xml_path)
-            error('Cannot find protocol @0@, file does not exist: @1@'.format(proto_name, xml_path))
+            if proto_optional
+                wayland_platform_c_args += ['-DHAVE_@0@=0'.format(proto_macro)]
+                continue
+            else
+                error('Cannot find protocol @0@, file does not exist: @1@'.format(proto_name, xml_path))
+            endif
+        endif
+
+        if proto_optional
+            wayland_platform_c_args += ['-DCOG_HAVE_@0@=1'.format(proto_macro)]
         endif
 
         wayland_platform_sources += [custom_target(