backend/x11: add support for adaptive_sync_enabled

This sets the _VARIABLE_REFRESH window property [1].

[1]: https://gitlab.freedesktop.org/mesa/mesa/blob/0616b7ac90cf4f86bb409d34101e3a3cceac8cbe/src/vulkan/wsi/wsi_common_x11.c#L1374

backend/x11/backend.c

@@ -203,6 +203,7 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 		{ .name = "WM_DELETE_WINDOW", .atom = &x11->atoms.wm_delete_window },
 		{ .name = "_NET_WM_NAME", .atom = &x11->atoms.net_wm_name },
 		{ .name = "UTF8_STRING", .atom = &x11->atoms.utf8_string },
+		{ .name = "_VARIABLE_REFRESH", .atom = &x11->atoms.variable_refresh },
 	};
 
 	for (size_t i = 0; i < sizeof(atom) / sizeof(atom[0]); ++i) {

backend/x11/output.c

@@ -118,6 +118,19 @@ static bool output_commit(struct wlr_output *wlr_output) {
 		}
 	}
 
+	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED &&
+			x11->atoms.variable_refresh != XCB_ATOM_NONE) {
+		if (wlr_output->pending.adaptive_sync_enabled) {
+			uint32_t enabled = 1;
+			xcb_change_property(x11->xcb, XCB_PROP_MODE_REPLACE, output->win,
+				x11->atoms.variable_refresh, XCB_ATOM_CARDINAL, 32, 1,
+				&enabled);
+		} else {
+			xcb_delete_property(x11->xcb, output->win,
+				x11->atoms.variable_refresh);
+		}
+	}
+
 	if (wlr_output->pending.committed & WLR_OUTPUT_STATE_BUFFER) {
 		pixman_region32_t *damage = NULL;
 		if (wlr_output->pending.committed & WLR_OUTPUT_STATE_DAMAGE) {

include/backend/x11.h

@@ -72,6 +72,7 @@ struct wlr_x11_backend {
 		xcb_atom_t wm_delete_window;
 		xcb_atom_t net_wm_name;
 		xcb_atom_t utf8_string;
+		xcb_atom_t variable_refresh;
 	} atoms;
 
 	// The time we last received an event