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Delphi. Remove a border of TabSheet of PageControl
unit Unit1;

interface

uses
  ...,
  CommCtrl;

type
  TPageControl = class(ComCtrls.TPageControl)
  private
    procedure TCMAdjustRect(var Msg: TMessage); message TCM_ADJUSTRECT;
  end;

  TForm1 = class(TForm)
    ...
  end;

...

procedure TPageControl.TCMAdjustRect(var Msg: TMessage);
begin
  inherited;
  if Msg.WParam = 0 then
    InflateRect(PRect(Msg.LParam)^, 4, 4)
  else
    InflateRect(PRect(Msg.LParam)^, -4, -4);
end;

...

end.