Update Y-sort position of the first item in the sorted subtree

doc/classes/CanvasItem.xml

@@ -600,7 +600,7 @@
 			[b]Note:[/b] For controls that inherit [Popup], the correct way to make them visible is to call one of the multiple [code]popup*()[/code] functions instead.
 		</member>
 		<member name="y_sort_enabled" type="bool" setter="set_y_sort_enabled" getter="is_y_sort_enabled" default="false">
-			If [code]true[/code], child nodes with the lowest Y position are drawn before those with a higher Y position. If [code]false[/code], Y-sorting is disabled. Y-sorting only affects children that inherit from [CanvasItem].
+			If [code]true[/code], this [CanvasItem] and its [CanvasItem] child nodes are sorted according to the Y position. Nodes with a lower Y position are drawn before those with a higher Y position. If [code]false[/code], Y-sorting is disabled.
 			You can nest nodes with Y-sorting. Child Y-sorted nodes are sorted in the same space as the parent Y-sort. This feature allows you to organize a scene better or divide it into multiple ones without changing your scene tree.
 		</member>
 		<member name="z_as_relative" type="bool" setter="set_z_as_relative" getter="is_z_relative" default="true">

servers/rendering/renderer_canvas_cull.cpp

@@ -306,12 +306,14 @@ void RendererCanvasCull::_cull_canvas_item(Item *p_canvas_item, const Transform2
 			child_item_count = ci->ysort_children_count + 1;
 			child_items = (Item **)alloca(child_item_count * sizeof(Item *));
 
+			ci->ysort_xform = ci->xform.affine_inverse();
+			ci->ysort_pos = Vector2();
+			ci->ysort_modulate = Color(1, 1, 1, 1);
+			ci->ysort_index = 0;
 			ci->ysort_parent_abs_z_index = parent_z;
 			child_items[0] = ci;
 			int i = 1;
 			_collect_ysort_children(ci, Transform2D(), p_material_owner, Color(1, 1, 1, 1), child_items, i, p_z);
-			ci->ysort_xform = ci->xform.affine_inverse();
-			ci->ysort_modulate = Color(1, 1, 1, 1);
 
 			SortArray<Item *, ItemPtrSort> sorter;
 			sorter.sort(child_items, child_item_count);