Preview windows are closed when they are no longer relevant, while preview buffers are killed if they have not been used for other purposes beside previewing. This list of buffers is relevant for as long as we are in the Dired buffer, otherwise all buffers therein are killed (buffers that were alive before being previewed are not touched). The new dired-preview-display-action-alist-below function has a straightforward behaviour: it always shows the preview below the current window and it always makes the preview window 0.3 times the height of the Emacs frame.