Hey Neil - I can't speak to the intended behavior, but it's not the behavior that I want or expect (nor how it behaved ~1 month ago).
To me the point of having the individual windows broken apart in the taskbar is that the previews aren't necessary. If they were all grouped together then the previews would serve great as a way to drill down into a specific one.
I just checked on a Win 10 machine, and there's some sort of logic that causes the previews equivalent (just a list of windows, no thumbnail) to appear on hover, but I'm not 100% sure how it decides whether to show one or not. It looks like if I click on one of the taskbar windows in a group (ie. one of the Chrome windows) then the previews stop appearing for the Chrome windows, even if I mouse out of the taskbar. If I mouse back into some other group of windows, then the previews start happening again. I can try to put together a video if that would be helpful.
Also, when they are showing on Win 10, they show vertically aligned, next to the taskbar so don't disrupt my ability to move the mouse to the taskbar item for the window I'd like to switch to.
As Start11 is right now without me applying my fix above, it's frustrating to try to switch between ~15 open chrome windows as the preview is horizontal and covers up a part of the taskbar that I'm trying to look at/pick from.
