Hi Jon,
We have checked the reported issue in the same online sample in our end with the mentioned chrome browser version (68.0.3440.84 (Official Build) (64-bit)) and the dialog is displaying properly without any z-index issue as shown in the following screenshots. Also, we have prepared the Angular sample and checked the same and dialog is displaying properly in that sample. Please refer to the working sample from the following location.
Screenshots:
Online Sample Screenshot:
Angular Sample Screenshot:
Kindly check with the above sample, and if you still face the same issue, kindly revert back to us with some more details like complete schedule rendering code or reproduce the mentioned issue in the above sample. The information provided by you will be more helpful for us to analyze the issue further and provide you the possible solution.
Regards,
Velmurugan