.h    class TForm1 : public TForm
{
__published:        // IDE-managed Components
        TPanel *Panel1;
        TTreeView *TreeView1;
        TMemo *Memo1;
private:        // User declarations
public:                // User declarations
        TWndMethod OldTTreeViewWndProc;
        void __fastcall TTreeViewWndProc(TMessage& Message);
        __fastcall TForm1(TComponent* Owner);
};    .cpp
const WM_DOCKCLOSE = 45067;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
    OldTTreeViewWndProc = TreeView1->WindowProc;
    TreeView1->WindowProc = TTreeViewWndProc;
}
//---------------------------------------------------------------------------    void __fastcall TForm1::TTreeViewWndProc(TMessage& Message)
{
    if (Message.Msg == WM_DOCKCLOSE)
    {
        ShowMessage("DOCKCLOSE");
    }
    OldTTreeViewWndProc(Message);
}