Fix invisible driver window issues in Handy Tech driver (#12618)
When vendor-specific applications need direct access to a Handy Tech Braille display, they indicate this by sending a window message to a driver-created invisible window. #9691 introduced this window for the handy tech driver, but it had some serious issues when doing the following
ensures that only one invisible driver window can be active by saving it on the class instead on instances of the class.
Co-authored-by: buddsean <sean@nvaccess.org>
