This is assuming you are using a list box as your drop-down ListControl:
The reason the combo-containing form gets deactivated is because when you mouse-down on the list box, it activates its parent form (the list box itself will be hosted on a borderless form).
You can work around this by deriving from a list box as follows and using it in place of the list box:
public class StaticListBox : ListBox
protected override void WndProc(ref Message m)
// Prevent the list box from processing the LeftMouse Down message,
// since it activates it's parent Form in it's Default Wnd Proc.
if(m.Msg == 0x201/*WM_LBUTTONDOWN*/)
Public Class StaticListBox Inherits ListBox
Protected Overrides Sub WndProc(ByRef m As Message)
' Prevent the list box from processing the LeftMouse Down message,
' since it activates it's parent Form in it's Default Wnd Proc.
If m.Msg = 0x201 Then
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