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ComboBox1.Perform (CB_SETDROPPEDWIDTH, 200 {PixelWidth}, 0);                    

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Procedure tForm1.ComboBox1DropDown (Sender: tObject);
Begin
  ComboBox1.Perform (CB_SETDROPPEDWIDTH, 200, 0);
End;                    

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Type
  tEnhComboBox = Class (tComboBox)
    Private
      fDropDownWidth : Integer;
    Protected
      procedure DropDown; override;
    Published
      Property DropDownWidth: Integer Read fDropDownWidth Write fDropDownWidth;
  End;                    

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Procedure tEnhCombo.DropDown;
Begin
  Inherited DropDown;

  If fDropDownWidth > 0 Then
    Self.Perform (CB_SETDROPPEDWIDTH, fDropDownWidth, 0)
End;