Question:
How do I get a ComboBox to drop down its list box in code?
Answer:
Send the ComboBox a CB_SHOWDROPDOWN message, passing a 1 (true) in
the WParam parameter to drop the list down, or a 0 (false) to hide it.
procedure TForm1.Button1Click(Sender: TObject);
begin
SendMessage(ComboBox1.Handle,
CB_SHOWDROPDOWN,
1,
0);
ComboBox1.SetFocus;
end;