xwayland: add wlr_xwayland_set_workareas()

This function allows compositors to set the _NET_WORKAREA property on
the root window. XWayland clients use _NET_WORKAREA to determine how
much of the screen is not covered by panels/docks. The property is used
for example by Qt to determine areas of the screen that popup menus
should not overlap (see QScreen::availableVirtualGeometry).

include/wlr/xwayland/xwayland.h

@@ -16,6 +16,7 @@
 #include <xcb/xcb_icccm.h>
 #include <wlr/util/addon.h>
 
+struct wlr_box;
 struct wlr_xwm;
 struct wlr_data_source;
 struct wlr_drag;
@@ -285,4 +286,14 @@ bool wlr_xwayland_or_surface_wants_focus(
 enum wlr_xwayland_icccm_input_model wlr_xwayland_icccm_input_model(
 	const struct wlr_xwayland_surface *xsurface);
 
+/**
+ * Sets the _NET_WORKAREA root window property. The compositor should set
+ * one workarea per virtual desktop. This indicates the usable geometry
+ * (relative to the virtual desktop viewport) that is not covered by
+ * panels, docks, etc. Unfortunately, it is not possible to specify
+ * per-output workareas.
+ */
+void wlr_xwayland_set_workareas(struct wlr_xwayland *wlr_xwayland,
+	const struct wlr_box *workareas, size_t num_workareas);
+
 #endif

include/xwayland/xwm.h

@@ -88,6 +88,7 @@ enum atom_name {
 	DND_ACTION_PRIVATE,
 	NET_CLIENT_LIST,
 	NET_CLIENT_LIST_STACKING,
+	NET_WORKAREA,
 	ATOM_LAST // keep last
 };
 

xwayland/xwm.c

@@ -90,6 +90,7 @@ static const char *const atom_map[ATOM_LAST] = {
 	[DND_ACTION_PRIVATE] = "XdndActionPrivate",
 	[NET_CLIENT_LIST] = "_NET_CLIENT_LIST",
 	[NET_CLIENT_LIST_STACKING] = "_NET_CLIENT_LIST_STACKING",
+	[NET_WORKAREA] = "_NET_WORKAREA",
 };
 
 #define STARTUP_INFO_REMOVE_PREFIX "remove: ID="
@@ -2327,3 +2328,24 @@ enum wlr_xwayland_icccm_input_model wlr_xwayland_icccm_input_model(
 	}
 	return WLR_ICCCM_INPUT_MODEL_NONE;
 }
+
+void wlr_xwayland_set_workareas(struct wlr_xwayland *wlr_xwayland,
+		const struct wlr_box *workareas, size_t num_workareas) {
+	uint32_t *data = malloc(4 * sizeof(uint32_t) * num_workareas);
+	if (!data) {
+		return;
+	}
+
+	for (size_t i = 0; i < num_workareas; i++) {
+		data[4 * i] = workareas[i].x;
+		data[4 * i + 1] = workareas[i].y;
+		data[4 * i + 2] = workareas[i].width;
+		data[4 * i + 3] = workareas[i].height;
+	}
+
+	struct wlr_xwm *xwm = wlr_xwayland->xwm;
+	xcb_change_property(xwm->xcb_conn, XCB_PROP_MODE_REPLACE,
+			xwm->screen->root, xwm->atoms[NET_WORKAREA],
+			XCB_ATOM_CARDINAL, 32, 4 * num_workareas, data);
+	free(data);
+}