DoDock

procedure TControl.DoDock(NewDockSite: TWinControl; var ARect: TRect);
begin
  { Erase TControls before UpdateboundsRect modifies position }
  if not (Self is TWinControl) then InvalidateControl(Visible, False);
  if Parent <> NewDockSite then
    UpdateBoundsRect(ARect) else
    BoundsRect := ARect;

{ The last line is the problem
Before call width is 154 and height is 774.
After call that set parent to nil
is width 249 and height is 554}
  if (NewDockSite = nil) or (NewDockSite = NullDockSite) then Parent := nil;
end;