Defining custom actions and changing keyboard shortcuts - tamius-han/ultrawidify GitHub Wiki
As of version 4.0.0, it's possible to set custom keyboard shortcuts and define custom actions. This can be done via 'Actions' page in extension preferences:

In order to add a new action, click 'Add new action' button. You will be greeted by a popup like this:

Action namedetermines the name of the action.Default labeldetermines how the action will be called in the popup.Show in popupdetermines whether the action will be shown in extension popup.+— click the big orange plus to choose what the action will do.
When you click the +, a dropdown will appear:

First, you select an action from the list. Once you select an action, you will get a dropdown with parameter options. Some parameters may require an additional argument that you need to enter manually:

Click 'save' to save what the chosen action will do. It's possible for action to do more than one thing at a time — just click the orange + again and repeat the procedure.

Once you're done, click 'save' to add your custom action.
Editing and removing actions
In general, 'x' removes things. Clicking pencil icon allows you to edit things.