您可以先將 ListBox1 設在 Panel1 上面，並將 ListBox1 的 Align 屬性設為 alClient，或者是經由程式碼來設定 (請見後附範例)。
    
在兩個 Panel 元件的 OnExit 或 UnDock 事件中設定 Panel 的 Width 為 0 或將 AutoSize 屬性設為 true 都可以在 UnDock 後將 DockSite 隱藏起來。
    
透過 DockSite (本例中的 Panel 元件) 的 OnGetSiteInfo 與 OnDockOver 事件可以偵測並繪製準備 Dock 的方框區域。
    
給您一個範例如下，程式稍長了一點點，因此偷個懶不對所有敘述都加上註解，如果對某段程式碼有疑問的話，再另外討論好了：

int PWIDTH = 120;  // ListBox1 崁入 DockSite 後的使用寬度    //---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
    // 將 ListBox1 起始位置崁在 Panel2 內
    Panel2->AutoSize = false;
    ListBox1->Dock(Panel2, TRect(0, 0, PWIDTH, Panel2->ClientHeight));
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel1GetSiteInfo(TObject *Sender,
      TControl *DockClient, TRect &InfluenceRect, TPoint &MousePos,
      bool &CanDock)
{
    TWinControl *Docksite = reinterpret_cast  (Sender);
    if (Docksite->DockClientCount < 1) {
        TPoint p = Docksite->Parent->ClientToScreen(TPoint(0,0));
        InfluenceRect = TRect(p.x, p.y, (p.x   PWIDTH), (p.y   Docksite->Parent->ClientHeight));
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel1DockDrop(TObject *Sender,
      TDragDockObject *Source, int X, int Y)
{
    TPanel *Docksite = reinterpret_cast  (Sender);
    Docksite->AutoSize = false;
    Docksite->Width = PWIDTH;
    Docksite->Parent->Realign();
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel1UnDock(TObject *Sender, TControl *Client,
      TWinControl *NewTarget, bool &Allow)
{
    TPanel *DockSite = reinterpret_cast  (Sender);
    if (DockSite->DockClientCount == 1) {
        DockSite->AutoSize = true;
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel1DockOver(TObject *Sender,
      TDragDockObject *Source, int X, int Y, TDragState State,
      bool &Accept)
{
    TWinControl *Docksite = reinterpret_cast  (Sender);
    if (Docksite->DockClientCount < 1) {
        TPoint p = Docksite->Parent->ClientToScreen(TPoint(0,0));
        Source->DockRect = TRect(p.x, p.y, (p.x   PWIDTH), (p.y   Docksite->Parent->ClientHeight));
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel2GetSiteInfo(TObject *Sender,
      TControl *DockClient, TRect &InfluenceRect, TPoint &MousePos,
      bool &CanDock)
{
    TWinControl *Docksite = reinterpret_cast  (Sender);
    if (Docksite->DockClientCount < 1) {
        TPoint p = Docksite->ClientToScreen(TPoint(0,0));
        InfluenceRect = TRect((p.x - PWIDTH), p.y, p.x, (p.y   Docksite->ClientHeight));
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel2DockOver(TObject *Sender,
      TDragDockObject *Source, int X, int Y, TDragState State,
      bool &Accept)
{
    if (Panel2->DockClientCount < 1) {
        TPoint p = Panel2->ClientToScreen(TPoint(0,0));
        Source->DockRect = TRect((p.x - PWIDTH), p.y, p.x, (p.y   Panel2->ClientHeight));
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel2DockDrop(TObject *Sender,
      TDragDockObject *Source, int X, int Y)
{
    TPanel *Docksite = reinterpret_cast  (Sender);
    Docksite->AutoSize = false;
    Docksite->Width = PWIDTH;
    Docksite->Parent->Realign();
}
//---------------------------------------------------------------------------    void __fastcall TForm1::Panel2UnDock(TObject *Sender, TControl *Client,
      TWinControl *NewTarget, bool &Allow)
{
    TPanel *DockSite = reinterpret_cast  (Sender);
    if (DockSite->DockClientCount == 1) {
        DockSite->AutoSize = true;
    }
}
//---------------------------------------------------------------------------    void __fastcall TForm1::ListBox1EndDock(TObject *Sender, TObject *Target,
      int X, int Y)
{
    if (Target != Panel1 && Panel1->DockClientCount < 1) {
        Panel1->Refresh();
    }
    if (Target != Panel2 && Panel2->DockClientCount < 1) {
        Panel2->Refresh();
    }
}
//---------------------------------------------------------------------------