Flickering TSpeedButton in a TMaskEdit descendant - delphi

Question

I try to create a TComboBox like component, inherited from TMaskEdit with a TSpeedButton inside the edit itself.
The problem when I type something into the edit the most of the button disappears (the right and the bottom edge still visible). If I move the mouse over the component or exit using TAB, the button appears again.
Here is the code:
interface
uses
Windows, Messages, SysUtils, Classes, Controls, Forms, Mask, Buttons;
type
TMyEdit = class(TCustomMaskEdit)
private
FButton: TSpeedButton;
protected
procedure CreateButton;
public
constructor Create(AOwner: TComponent); override;
procedure CreateWnd; override;
destructor Destroy; override;
end;
implementation
constructor TMyEdit.Create(AOwner: TComponent);
begin
inherited;
CreateButton;
end;
procedure TMyEdit.CreateButton;
begin
FButton := TSpeedButton.Create(Self);
FButton.Parent := Self;
FButton.Align := alRight;
FButton.Width := 16;
FButton.Caption := '';
FButton.Transparent := False;
end;
destructor TMyEdit.Destroy;
begin
FreeAndNil(FButton);
inherited;
end;
procedure TMyEdit.CreateWnd;
begin
inherited;
Perform(EM_SETMARGINS, EC_RIGHTMARGIN, (FButton.Width + 4) shl 16);
end;
What do I miss?

Answer 1

Solved.
WS_CLIPCHILDREN flag have to be included in the CreateParams() AND the button must be placed on TWinControl descendant (TPanel in my case) OR the button itself must be a TWinControl descendant (for example TButton). Graphic controls has no Handle, thats the problem.
Modified code:
procedure TMyEdit.CreateButton;
var
xDrawRect: TRect;
xPanel : TPanel;
begin
xPanel := TPanel.Create(Self);
xPanel.Parent := Self;
xPanel.SetBounds(Width - Height, 0, Height, Height);
xPanel.BevelOuter := bvNone;
FButton := TSpeedButton.Create(Self);
FButton.Parent := xPanel;
FButton.Align := alClient;
FButton.Caption := '';
end;
procedure TMyEdit.CreateParams(var Params: TCreateParams);
begin
inherited CreateParams(Params);
Params.Style := Params.Style or WS_CLIPCHILDREN;
end;