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)
Super + W
under Ubuntu 16.04 with Unity. It provides you with a tile-style view of all open windows. As you are using Ubuntu 17.10 with the Unity-like Gnome environment, the key might be different. Under Gnome you may as well just presSuper
to get a tile-like view of all open applications. BTW: Super = Windows Key