Why doesn't this Synchronize procedure lock the main thread - multithreading

Question

I have following code:
TThread.Synchronize(nil,
procedure
begin
with Scope.New(TManualCaptchaForm.Create(img)) do
if It.ShowModal() = mrOk then
res := It.edtResolved.Text;
end
);
Why does the form appear several times when multiple TThreads use this procedure for synchronization? I know a workaround, and there is nothing unusual (e. g. no other "hand-made" ways to sync with main thread), but why am I not experiencing a lock?
Yes, Scope.New is kinda smart-pointer, BUT only i see TThread.Synchronize and passed closure? Documentation says that any method/closure passed to TThread.Synchronize will be execute inside main thread. Obviously, ShowModal must block main thread, but it didn't do that. As for me, it's very strange that any other window start behave as main thread and pump synchronization queue.
P. s. almost MVP:
TThread.Synchronize(nil,
procedure
var Form: TForm1;
begin
Form := TForm1.Create(nil);
try
Form.ShowModal();
finally
Form.Free;
end;
end
);
Run this code in 2+ threads and see bug. Anyway, now I know that synchronization queue pumped by any window message loop, not just by main form.
Btw, my question was "Why TThread.Synchronize behave so unclear/not logically?", not about my own code.

Answer 1

The fundamental misunderstanding here is that because .ShowModal is a blocking call you expect that it also suspends message processing. It does not. When you create a modal window the modal window takes over message processing - it must do this or the window would not function. The main thread is still processing the message loop, it is just doing it in a different context.
If you want to think of it this way, ShowModal behaves a lot like Application.ProcessMessages. This has nothing to do with Synchronize. If you examine the code for ShowModal you find :
{ ... }
Show;
try
SendMessage(Handle, CM_ACTIVATE, 0, 0);
ModalResult := 0;
{ *** Here is your message loop *** }
repeat
Application.HandleMessage;
if Application.Terminated then ModalResult := mrCancel else
if ModalResult <> 0 then CloseModal;
until ModalResult <> 0;
{ *** ------------------------- *** }
Result := ModalResult;
SendMessage(Handle, CM_DEACTIVATE, 0, 0);
if GetActiveWindow <> Handle then ActiveWindow := 0;
finally
Hide;
end;
{ ... }
If you want to prevent reentrance here you have to devise your own explicit method to do so.