Forcing a child form with CreateParam's Params.WndParent? - delphi

Question

In some legacy D7 code I inherited and am migrating to XE5, I found the code below.
The comment states it's tricking windows into thinking it's a child form if it's created from a non-WinControl. There is one place in the code base where Create is called with AOwner as nil. (A form is available at the time of that call, so not sure why they did that...)
Any suggestions as to what the programmer's goal was?
private
FParentWinControl: TWinControl; {Don't mess with! Used in CreateParams}
procedure TFormX.CreateParams(var params: TCreateParams); override;
public
constructor TFormX.Create( AOwner: TComponent); reintroduce;
end;
constructor TFormX.Create( AOwner: TComponent);
begin
if AOwner IS TWinControl then
FParentWinControl := TWinControl(AOwner)
else
FParentWinControl := NIL;
inherited Create(AOwner);
end; { Create }
procedure TFormX.CreateParams(var params: TCreateParams);
begin
inherited CreateParams(params);
if (NOT fCreateParamsHasBeenRun) then
begin
fCreateParamsHasBeenRun := TRUE;
if Assigned(FParentWinControl) then
Params.WndParent := FParentWinControl.Handle; {tricks windows into thinking it's a child form}
end;
end;

Answer 1

This code predates and loosely mimics the PopupMode and PopupParent properties that were added to TCustomForm in Delphi 8. Assuming AOwner is another Form, use those properties in modern Delphi versions, eg:
constructor TFormX.Create( AOwner: TComponent);
begin
inherited Create(AOwner);
if AOwner Is TCustomForm then
PopupParent := TCustomForm(AOwner);
end;
Also, the use of fCreateParamsHasBeenRun was wrong. CreateParams() is called every time the Form's window is (re)created, so the WndParent needed to be applied every time, not conditionally. If you need to keep the CreateParams() logic (such as if AOwner is a non-TCustomForm windowed control), you need to remove fCreateParamsHasBeenRun.