When you initialize a Scheduler that is in a hidden part of a page (like in case of a hidden tab) it is not possible to initialize the dimensions properly (all dimensions returned by the browser are 0 pixels).
The latest version of DayPilot autodetects when the Scheduler becomes visible and tries to fix the layout.
If it doesn't work you can try calling dp.show() when the tab becomes visible. See also:
http://api.daypilot.org/daypilot-scheduler-show/
This should correct the display.