layer shell: add wlr_layer_surface_v1_popup_surface_at()

This function will allow compositors to implement input handling in a
way consistent with rendering more easily.

Calling wlr_layer_surface_v1_surface_at() and checking if the result is
a wlr_xdg_popup is flawed as there may be subsurfaces in the popup tree.

include/wlr/types/wlr_layer_shell_v1.h

@@ -151,4 +151,13 @@ struct wlr_surface *wlr_layer_surface_v1_surface_at(
 		struct wlr_layer_surface_v1 *surface, double sx, double sy,
 		double *sub_x, double *sub_y);
 
+/**
+ * Find a surface within this layer-surface's popup tree at the given
+ * surface-local coordinates. Returns the surface and coordinates in the leaf
+ * surface coordinate system or NULL if no surface is found at that location.
+ */
+struct wlr_surface *wlr_layer_surface_v1_popup_surface_at(
+		struct wlr_layer_surface_v1 *surface, double sx, double sy,
+		double *sub_x, double *sub_y);
+
 #endif

types/wlr_layer_shell_v1.c

@@ -590,6 +590,17 @@ void wlr_layer_surface_v1_for_each_popup(struct wlr_layer_surface_v1 *surface,
 struct wlr_surface *wlr_layer_surface_v1_surface_at(
 		struct wlr_layer_surface_v1 *surface, double sx, double sy,
 		double *sub_x, double *sub_y) {
+	struct wlr_surface *sub = wlr_layer_surface_v1_popup_surface_at(surface,
+			sx, sy, sub_x, sub_y);
+	if (sub != NULL) {
+		return sub;
+	}
+	return wlr_surface_surface_at(surface->surface, sx, sy, sub_x, sub_y);
+}
+
+struct wlr_surface *wlr_layer_surface_v1_popup_surface_at(
+		struct wlr_layer_surface_v1 *surface, double sx, double sy,
+		double *sub_x, double *sub_y) {
 	struct wlr_xdg_popup *popup_state;
 	wl_list_for_each(popup_state, &surface->popups, link) {
 		struct wlr_xdg_surface *popup = popup_state->base;
@@ -606,5 +617,5 @@ struct wlr_surface *wlr_layer_surface_v1_surface_at(
 		}
 	}
 
-	return wlr_surface_surface_at(surface->surface, sx, sy, sub_x, sub_y);
+	return NULL;
 }