In the icons-only task manager, if I have multiple instances of an application and they are grouped, when I hover the mouse, I get a preview of the windows above the icon in a horizontal layout (a row), similar to MS windows. Would be nice to have the option to make those small windows previews appear in a vertical layout (i.e. in a column instead of a row) instead of a horizontal layout.
Would you do that to solve anything or purely a visual preference?
Doesn't "solve" anything, it's just a preference. I find a vertical layout more ergonomic (like when right-clicking for recent files), and I also think would look more visually appealing.
From a usability perspective, making the list vertical would actually be slightly faster for expert mouse users, because you wouldn't have to make a change-of-direction with your cursor to select a window when there are more than 2 in the list. But I suspect this would attract complaints that it stuck out too far and was ugly. Probably not worth exchanging one category of complains for another; vote to not implement.
The main argument I could see for vertical is that our default Alt+Tab UI is a vertical sidebar and it might be more consistent that way. I have no preference personally, except I really don't want to add an option for changing it.
Don't think this is gonna happen, sorry.