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.