desktop: add means of grabbing the surface at (x,y)

desktop/output.c

@@ -31,6 +31,28 @@
 #include "util.h"
 #include "view.h"
 
+void
+cage_output_surface_at(struct cg_output *output, double lx, double ly, struct wlr_surface **surface, double *sx,
+		       double *sy)
+{
+	struct cg_view *view;
+	wl_list_for_each (view, &output->views, link) {
+		// TODO: this only works because all views are currently maximized on a single output.
+		// When multi-monitor setups and dialogs are added, this will need extending.
+		double view_sx = lx;
+		double view_sy = ly;
+
+		double _sx, _sy;
+		struct wlr_surface *_surface = cage_view_wlr_surface_at(view, view_sx, view_sy, &_sx, &_sy);
+		if (_surface != NULL) {
+			*sx = _sx;
+			*sy = _sy;
+			*surface = _surface;
+			break;
+		}
+	}
+}
+
 void
 cage_output_damage_whole(struct cg_output *output)
 {

desktop/output.h

@@ -25,6 +25,11 @@ struct cg_output {
 	struct wl_list link; // cg_server::outputs
 };
 
+/**
+ * Iterates over all the views on an output and attempts to find the surface under the cursor.
+ */
+void cage_output_surface_at(struct cg_output *output, double lx, double ly, struct wlr_surface **surface, double *sx,
+			    double *sy);
 void cage_output_damage_whole(struct cg_output *output);
 void cage_output_damage_region(struct cg_output *output, struct wlr_box *region);
 void cage_output_damage_surface(struct cg_output *output, struct wlr_surface *surface, int sx, int sy);

desktop/view.c

@@ -112,6 +112,17 @@ cage_view_damage_part(struct cg_view *view)
 	cage_view_for_each_surface(view, damage_surface_iterator, NULL);
 }
 
+struct wlr_surface *
+cage_view_wlr_surface_at(struct cg_view *view, double sx, double sy, double *sub_x, double *sub_y)
+{
+	assert(view != NULL);
+	assert(sub_x != NULL);
+	assert(sub_y != NULL);
+	assert(view->impl->wlr_surface_at != NULL);
+
+	return view->impl->wlr_surface_at(view, sx, sy, sub_x, sub_y);
+}
+
 void
 cage_view_activate(struct cg_view *view, bool activate)
 {

desktop/view.h

@@ -74,6 +74,15 @@ struct cg_view_impl {
 	 * Activate a view.
 	 */
 	void (*activate)(struct cg_view *view, bool activate);
+
+	/**
+	 * XDG toplevels may have nested surfaces, such as popup windows
+	 * for context menus or tooltips. This tests if any of those are
+	 * underneath the coordinates lx and ly (in output Layout Coordinates).
+	 * If so, it sets the surface pointer to that wlr_surface and the
+	 * sx and sy coordinates to the coordinates relative to that
+	 * surface's top-left corner. */
+	struct wlr_surface *(*wlr_surface_at)(struct cg_view *view, double sx, double sy, double *sub_x, double *sub_y);
 };
 
 bool cage_view_is_primary(struct cg_view *view);
@@ -82,6 +91,7 @@ void cage_view_for_each_surface(struct cg_view *view, wlr_surface_iterator_func_
 char *cage_view_get_title(struct cg_view *view);
 void cage_view_damage_whole(struct cg_view *view);
 void cage_view_damage_part(struct cg_view *view);
+struct wlr_surface *cage_view_wlr_surface_at(struct cg_view *view, double sx, double sy, double *sub_x, double *sub_y);
 void cage_view_activate(struct cg_view *view, bool activate);
 bool cage_view_is_mapped(struct cg_view *view);
 void cage_view_unmap(struct cg_view *view);

desktop/xdg_shell.c

@@ -66,6 +66,13 @@ get_title(struct cg_view *view)
 	return xdg_shell_view->xdg_surface->toplevel->title;
 }
 
+static struct wlr_surface *
+wlr_surface_at(struct cg_view *view, double sx, double sy, double *sub_x, double *sub_y)
+{
+	struct cg_xdg_shell_view *xdg_shell_view = xdg_shell_view_from_view(view);
+	return wlr_xdg_surface_surface_at(xdg_shell_view->xdg_surface, sx, sy, sub_x, sub_y);
+}
+
 static void
 activate(struct cg_view *view, bool activate)
 {
@@ -153,6 +160,7 @@ static const struct cg_view_impl xdg_shell_view_impl = {
 	.is_primary = is_primary,
 	.get_title = get_title,
 	.activate = activate,
+	.wlr_surface_at = wlr_surface_at,
 };
 
 void