Sometimes an ultrawide monitor is just... too wide. You can adjust Jwno's top-level frames, so that they show your managed windows around the center of your screen. This can be achieved in multiple ways.

You can :transform a top-level frame directly:

(import jwno/util)

(def top-frame (:get-current-top-frame (get-in jwno/context [:window-manager :root])))
(def rect (in top-frame :rect))

# This will reserve 500 pixels of space on the left and right sides of your monitor.
(:transform top-frame (util/shrink-rect rect {:left 500 :right 500 :top 0 :bottom 0}))
(:retile (in jwno/context :window-manager) top-frame)

# To restore the layout that fills the whole screen:
(:transform top-frame (get-in top-frame [:monitor :work-area]))
(:retile (in jwno/context :window-manager) top-frame)

Or, you can use frame paddings to do the same thing automatically:

(import jwno/util)

(defn is-my-ultrawide-monitor? [monitor]
  (def [width height] (util/rect-size (in monitor :rect)))
  (>= (/ width height) (/ 21 9)))

(:add-hook (in jwno/context :hook-manager) :monitor-updated
   (fn [top-frame]
     (when (is-my-ultrawide-monitor? (in top-frame :monitor))
       (put (in top-frame :tags) :paddings {:left 500 :right 500 :top 10 :bottom 10}))))

But this second method only takes effect when a new monitor is detected, or when your monitor configuration is actually changed, e.g. you set it to a different DPI or resolution. It will also override your frame's :padding setting.