UNMAINTAINED
GNOME Shell extension. Supports GNOME Shell 3.22.
Extension page: https://extensions.gnome.org/extension/516/force-thumbnails-on-alt-tab/
Github: https://github.com/dsboger/gs-ext-alttabmod-thumbnails
Default Alt+Tab behavior is to wait around half a second before showing window thumbnails or not to show thumbnails at all for single window applications. This extension forces Alt+Tab popup to always show window thumbnails of the selected app. This way you can always preview the window you are jumping to.
OBS: not guaranteed to work together with other extensions that also modify Alt+Tab popup.