During the resize event, make a call to the Windows API function InvalidateRect(). Passing nil as the second parameter will cause the clipping rect of the form to include the entire client area of the form. The third parameter denotes if the background of the form should be erased and repainted. Simply calling the invalidate method of the form may result in a clipping region and possibly the background getting erased. Example:

procedure TForm .FormResize(Sender: TObject);
begin
  InvalidateRect(Form .Handle, nil, false);
end;