I do have an issue with slideshow crossfades using a second display (windows 10). I just switched from LR 4 to LR 6 (never had LR 5). In 4 I always connected a beamer or second monitor in just activating it through the windows system control interface / desktop. In the slide show settings I configured to play the slide show on the 2nd display and everything went well. With LR 6 I have the following effect: if I start the slide show exactly in this way the desktop on the 2nd display disappears and gets black. If I activate the 2nd display using the "second window button" in the film strip and set the view option on the 2nd window to "slide show" the slide show works principally - but without cross fades. Images are switched synchronously to the main window.
What am I doing wrong? What's the correct way to present a slide show on a 2nd display (monitor or beamer) with crossfades?