You can open the preview window by pressing p.

Preview window displays output of the jj show (or jj op show) command of the selected revision/file/operation.

You can specify extra arguments you want to be passed to the jj show command in the preview.extra_args of the configuration file.

For example, to show the summary along with the diff then you can use the following configuration:

[preview]
extra_args = ["--summary", "--git"]

If the currently selected item is a file, then jj diff output is displayed.

While the preview window is showing, you can press; ctrl+n to scroll one line down, ctrl+p to scroll one line up, ctrl+n to scroll half page down, ctrl+u to scroll half page up.

By default preview window has the following configuration:

[keys.preview]
  mode = ["p"]
  scroll_up = ["ctrl+p"]
  scroll_down = ["ctrl+n"]
  half_page_down = ["ctrl+d"]
  half_page_up = ["ctrl+u"]
  expand = ["ctrl+h"]
  shrink = ["ctrl+l"]
[preview]
  extra_args = []
  show_at_start = false
  width_percentage = 50.0
  width_increment_percentage = 5.0