To reproduce the problem, we need 2 Monitors with different scaling.
Let's say that Monitor 1 has 150% Scaling and Monitor 2 has 100% (Monitor 2 is the main monitor). When starting the program and click on radbutton1, then an empty form (Toolbox) is shown. Move the Toolbox form to the other monitor and then open the RadDropDownList. The problem is that the drop-down is shown not under the control.
Please refer to the attached project.
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.