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type
  TTree = class(TTreeView)
  private
    procedure CMVisibleChanged(var Message: TMessage); message CM_VISIBLECHANGED;
 end;

  TOnExpandedEvent = procedure(Sender: TObject; Node: TTreeNode) of object;

  TFlatTreeComboBox = class(TCustomComboBox)
    Tree : TTree;
  private
    OnExpandedEvent: TOnExpandedEvent;
  protected
    procedure TreeExpanded(Sender: TObject; Node: TTreeNode);  public
  public  
    constructor Create (AOwner: TComponent); override;
  published
    Property OnExpanded: TOnExpandedEvent read OnExpandedEvent write OnExpandedEvent;
  end;  

implementation

procedure TTree.CMVisibleChanged(var Message: TMessage);
begin
  Showmessage('test');
end;

constructor TFlatTreeComboBox.Create (AOwner: TComponent);
begin
  inherited Create(AOwner);
  ...
  Tree.OnExpanded:=Self.TreeExpanded;
end;

procedure TFlatTreeComboBox.TreeExpanded(Sender: TObject; Node: TTreeNode);
begin
  if Assigned(Self.OnExpanded) then Self.OnExpanded(Sender, Node);
end;
end.					

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type
  TOnVisibleChangedEvent = Proedure (Sender: TObject) of object;

  TTree = class(TTreeView)
    fOnVisibleChanged: TOnVisibleChangedEvent;
  private
    procedure CMVisibleChanged(var Message: TMessage); message CM_VISIBLECHANGED;
  published
    property OnVisibleChanged: TOnVisibleChangedEvent read fOnVisibleChanged write fOnVisibleChange;
end;

{...}

procedure TTree.CMVisibleChanged(var Message: TMessage); message CM_VISIBLECHANGED;
begin
  if assigned (fOnVisibleChanged) Then
    fOnVisibleChanged (Self);
end;					

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TTree = class(TTreeView)
  private
    OnVisibleChanged: TOnVisibleChangedEvent;
    FMouseInControl: Boolean;
    procedure CMVisibleChanged(var Message: TMessage); message CM_VISIBLECHANGED;
    ...
  protected
    ...
  published
  end;

 TFlatTreeComboBox = class(TCustomComboBox)
    Tree : TTree;
  private
    OnVisibleChangedEvent: TreeOnVisibleChangedEvent;
  protected
    procedure WMLButtonDown(var Message: TWMLButtonDown); reintroduce; message WM_LBUTTONDOWN;
    procedure TreeVisibleChanged(Sender: TObject);
  public
    constructor Create (AOwner: TComponent); override;
    destructor Destroy; override;
  published
    property OnVisibleChanged: TreeOnVisibleChangedEvent read OnVisibleChangedEvent write OnVisibleChangedEvent;
  end;

implementation

procedure TTree.CMVisibleChanged(var Message: TMessage);
begin
  //if assigned (OnVisibleChanged) Then OnVisibleChanged (Self);
end; 

procedure TFlatTreeComboBox.WMLButtonDown(var Message: TWMLButtonDown);
var
  CP, SP: TPoint;
  i, j: integer;
begin
  CP.X := Left;
  CP.Y := Top + Height;
  SP := Parent.ClientToScreen(CP);
  If HeightIndex = 0 then THeight:=TreeHeight;
  with Tree do
  begin
    Left := SP.X;
    Top := SP.Y;
    Width := Self.Width;
    if Enabled = False then
    begin
      if Tree.Items.Count = 0 then Tree.Height := Tree.GetItemHeight
      else
      begin
        j:=0;
        for i:=0 to Tree.Items.Count-1 do
        begin
          if Tree.Items[i].IsVisible = true then j:=j+1;
        end;
        if 5+j*Tree.GetItemHeight <= THeight then Tree.Height:=5+j*Tree.GetItemHeight
        else Tree.Height:=THeight;
      end;
      Enabled := True;
      Visible := True;
      SetCaptureControl(Tree);
    end
    else begin
      Enabled := False;
      Visible := False;
      ReleaseCapture;
    end;
  end;
  PaintButton;
  HeightIndex:=1;
end;