I do this as well and I use the following routine to make it happen:
procedure TMyForm.PlaceInsideContainer(Container: TWinControl);
begin
Parent := Container;
Align := alClient;
BorderIcons := [];
BorderStyle := bsNone;
ParentBackground := True;
Show;
end;
I have no problems with this. The only difference that I could possibly imagine could be relevant is the assignment of BorderIcons, but I would doubt that causes a problem.