Hello ,
The zoder is for organizing widgets within the widget itself. This feature does not extend globally to all widget blueprints. To determine what widget blueprint will be on top you will need to know which was added to the viewport last. This means that another workaround could be to remove the widget that you want on top and to immediately add it back when ever another widget blueprint is added. With that being said there is a feature request that has been implemented on an internal build that will allow for more control over the layering of widgets. This feature will be available in a later release of the engine. I hope that this information helps.
Make it a great day