The problem was first discovered in a PB10 app but I verified it happens in PB 2017. I am on Windows 10.
The main window of the app is MDIHelp. Clicking a button opens a response window. Clicking a button on the response window opens another response window.
If I close the second response window by clicking the X button in the upper right corner, the first response window is moved behind the main window and becomes unresponsive.
I unchecked the ControlMenu property and that solved it.
Did I understand well that you open the first response straight from the mdi frame window itself? So there's no sheet from where you open it? If so, you could try to have an invisible sheet window and open the first response window from that one. Think I did something like that long long time ago, so not sure if it would work.
Just an idea, but I have the feeling that this incorrect behaviour in windows 8 and 10 (now also windows 7? since they push back all the "improvements") might have something to do with windows "not wanting to bother users with windows popping up (unless it's windows itself bothering you)". You can see that, when for example you want to show an address list for email when using OLE and outlook: The icon which represents the window of the address book "stays flashing" in the task bar and you have to first notice that and then click on it to see the window (the only way to get around that one is to use mapi for your addressbook and ole for the rest).
My wild guess is that maybe windows does not interfere as much if your response window has no red-cross-control to close it. Who knows how long this will still work.
Kind regards,
MiguelL