Correct window_rect.y with hide_edge_borders

With `hide_edge_borders both` (or at least `vertical`),
`window_rect.y` will equal `border_thickness` for SOME windows,
but it will be 0 for windows adjacent to top screen edge.

Therefore setting it to `border_thickness` is not sufficient.

This commit changes it to the actual y offset of content
into the container.

sway/ipc-json.c

@@ -577,7 +577,7 @@ static void ipc_json_describe_view(struct sway_container *c, json_object *object
 
 	struct wlr_box window_box = {
 		c->pending.content_x - c->pending.x,
-		(c->current.border == B_PIXEL) ? c->current.border_thickness : 0,
+		(c->current.border == B_PIXEL) ? c->pending.content_y - c->pending.y : 0,
 		c->pending.content_width,
 		c->pending.content_height
 	};