Some flushing method is needed for TFileStream

https://forum.lazarus.freepascal.org/index.php/topic,58619.msg436692.html#msg436692 This is rationale. Ie the reason for such a method.

TFileStream can be flushed on Win32 in Delphi: https://stackoverflow.com/questions/787019/how-to-flush-a-tfilestream

This method uses Win32 API function. So I suggest to add the virtual method to TFileStream (or TStream?) which will be implemented on Win32 (at least). Using Win32 API.

Alternative solution: change TJsonConfig.Flush like this:

procedure TJSONConfig.Flush;
....
        if S<>'' then
        begin
          F.WriteBuffer(S[1],Length(S));

          {$ifdef windows}
          Windows.FlushFileBuffers(F.Handle);
          {$endif}
        end;