for P := 0 to StatusBar.Panels.Count - 1 do
    begin
   StatusBar.Panels.Items[P].Width :=
   StatusBar.Canvas.TextWidth(StatusBar.Panels.Items[P].Text);
    end;

for P := 0 to StatusBar.Panels.Count - 1 do
    begin
   StatusBar.Panels.Items[P].Width :=
   StatusBar.Canvas.TextWidth(StatusBar.Panels.Items[P].Text)+
   2*GetSystemMetrics(SM_CXEDGE);
    end;

function TrueFontWidth(fnt: TFont; const text:string): Integer;
var
   dc: hdc;
   tsize : Windows.TSize;
begin
   dc := GetDC(0);
   SelectObject(DC, fnt.Handle);
   GetTextExtentPoint32(dc, PChar(text), Length(text), tsize);
   ReleaseDC(0, DC);
   Result := tsize.cx;
end;

procedure AutoSizeStatusbarPanel(sb: TStatusBar; idx:Integer);
var
   s : string;
   borders : array[0..2] of Integer;
begin
   // don't deal with simple panels
   if sb.SimplePanel
      // don't resize the last panel
      or (idx >= sb.Panels.Count-1) then
      Exit;

   // get the borders of the statusbar
   // border[0] = width of the horizontal border
   // border[1] = width of the vertical border
   // border[2] = width of the border between rectangles
   SendMessage(sb.Handle, SB_GETBORDERS, 0, Integer(@borders));

   s := sb.Panels[idx].Text;

   // calculate the width of the Panel
   sb.Panels[idx].Width := TrueFontWidth(sb.Font, s) +
      borders[2]*2 +2; // vertical border * 2 + 2 extra Pixels
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  iPanel: Word;
begin
  With StatusBar1 do
  begin
    Canvas.Font := Font; // <-- Dem Canvas die Font zuweisen:
    for iPanel := 0 to Panels.Count - 1 do
    begin
      Panels.Items[iPanel].Width := Canvas.TextWidth(Panels.Items[P].Text) + 5;
    end;
  end;
end;