Author: Jonas Bilinkevicius

I want to know how to prevent an undocked control from destroying when I clicked on 
the X shaped close button on the right corner.

Answer:

You have to create your own docksite form:

1   type
2     TNoCloseDockform = class(TCustomDockForm)
3     private
4       procedure WMSysCommand(var msg: TWMSyscommand); message WM_SYSCOMMAND;
5     end;
6   
7     { TNoCloseDockform }
8   
9   procedure TNoCloseDockform.WMSysCommand(var msg: TWMSyscommand);
10  begin
11    if (msg.CmdType and $FFF0) = SC_CLOSE then
12      Msg.result := 0
13    else
14      inherited;
15  end;
16  
17  procedure TForm1.FormCreate(Sender: TObject);
18  begin
19    panel1.FloatingDockSiteClass := TNoCloseDockform;
20  end;