I am not aware of any window-preview-on-mouse-hover feature, but you can activate previews of open windows on
Primary mouse button click by running
gsettings set org.gnome.shell.extensions.dash-to-dock click-action 'previews'
shift+click by running
gsettings set org.gnome.shell.extensions.dash-to-dock shift-click-action 'previews'
Middle mouse button click by running
gsettings set org.gnome.shell.extensions.dash-to-dock middle-click-action 'previews'
shift+middle mouse button click by running
gsettings set org.gnome.shell.extensions.dash-to-dock shift-middle-click-action 'previews'
(To find out all possible options for an action, run
gsettings range org.gnome.shell.extensions.dash-to-dock click-action
and so.)
As a side note, a GNOME extension called Dash to Panel puts the application icons to the top bar (and provides an option to move top bar to bottom):

It also provides the window-preview-on-mouse-hover feature:

(source: official GitHub page)