My scenario is that I have two Panels which changes Size on mouseleave/ mouseEnter events. One panel has a dropdownlist control. When popup is opened and Panel1 changes size upon Mouseenter event, the popup does not change its location. I've tried changing the popup Left property but doesn't fix the issue. Please see attached sample application.

Workaround:

        private void RadPanel1_MouseLeave(object sender, EventArgs e)
        {
            this.radDropDownList1.DropDownAnimationEnabled = false;
            this.radDropDownList1.DropDownListElement.ClosePopup();
            radPanel1.Size = new Size(30, 400);
            radPanel2.Left = radPanel1.Right + 30;
            this.radDropDownList1.DropDownListElement.ShowPopup();
        }

        private void RadPanel1_MouseEnter(object sender, EventArgs e)
        {
            this.radDropDownList1.DropDownAnimationEnabled = false;
            this.radDropDownList1.DropDownListElement.ClosePopup();
            radPanel1.Size = new Size(200, 400);
            radPanel2.Left = radPanel1.Right + 30;
            this.radDropDownList1.DropDownListElement.ShowPopup();
        }

I am using Visual Studio 2019, version 16.3.1 and when using the DropDownList and running it in the VS debugger, the DropDownList get populated properly, but when you click the down arrow for the first time, the list drops down and you can scroll with the mouse wheel, but it will not allow you to select anything with the mouse.

If you click the down arrow to close the list, and click it again to reopen the list, it works properly - selecting the item chosen.

I have tried numerous property setting in attempt to narrow down if any of the properties set are causing it, but have been unsuccessful.

I have created several test projects with only 1 form and only the dropdownlist on the form and this happens reliably in every project I have tested within Visual Studio.

If I build the executable and run it directly, the DropDownList works properly as expected.

The DropDownList version is listed as 2019.3.917.40 in Visual Studio 2019 16.3.1.

I opened the project in JetBrains Rider on the same machine and everything works fine.