Grouping - how to reach thumbnails when selected "open last window if inactive"

I'm not sure I understand the question. The "Open last window if inactive, otherwise open thumbnail preview" option doesn't affect hovering, you still should be able to hover to show the thumbnails.

OK I see. 

The issue is, if the window is inactive, there is no easy way to reach the group thumbnails - I have to click the icon twice and with a bit of delay between the clicks which is very distracting.

I can see no reason why hover will not show the thumbnails in all cases (active or not etc). Also/alternatively perhaps the delay can be removed so that double clicking will show the thumbnails.

Thank you for great software :)

I can see no reason why hover will not show the thumbnails in all cases (active or not etc).

The thumbnails should be shown even when the program is not active. If it doesn't, perhaps it has something to do with your settings. Check your "Hovering" option in the tweaker, perhaps it's set to "Nothing"?

got it thx!