The z-index of RadMenu is calculated dynamically but the default value is 8000. So if you would like to prevent your custom drop-down of hiding it by RadMenu, the z-index should be bigger than 8000. If the issue occurs, we would like to ask you to open a support ticket where you will be able to attach a sample project where the issue is reproduced. So we will be able to test it locally and assist you in more efficient way.
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