Invisible dialogs

If you switch app while you have a modal dialog open, and the new window covers that dialog, then when you come back, that dialog is invisible but it's window proc still active. Ergo you can't exit the dialog because the UI is gone, and you can't exit the app since you can't access it's menu while a modal dialog is open.

This is a Z-order problem, which is obvious if you let the dialog be only partially covered by it's owner window.

To repeat: Open e.g Notepad, and open the Font dialog. Drag it so it's surrounded by it's owner. Switch to e.g Explorer and size/drag so it covers the dialog. Activate Notepad via it's taskbar button.. the dialog is gone.

Detected in r54606.