DevExpress is a third party tool that we often use in .net and that have special tools provided.
So , sometimes while using DevExpress Dropdown on popup , when we click on dropdown the items gets hidden behind the popup For that issue we have to add this line
DevExpress.XtraEditors.Popup.PopupBaseForm.ForceRemotingCompatibilityMode = true;
We can add this line in our page load or wherever you are facing the issue. This will set z-index of item globally and thus will help us resolve the issue.
So , this is how we can fix DevExpress Dropdown Items Overlapped by Popup