The "allowDefaultContextMenu" property lets you enable the default browser context menu. By default, this is disabled for the Scheduler so that you don't see the browser context menu over the Scheduler context menu on right click. Normally, you don't want to display the browser context menu at all.
You can disable the event context menu using "eventRightClickHandling" property:
dp.eventRightClickHandling = "Disabled";
The default value is "ContextMenu".
See also:
https://api.daypilot.org/daypilot-scheduler-eventrightclickhandling/
For a full list of properties that will let you make the Scheduler read-only please see the following tutorial:
https://code.daypilot.org/65945/javascript-scheduler-read-only-mode