Bug Report

• Package version(s): 5.20.3
• Browser and OS versions: all

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix? Yes, see #3504

Requested priority: Normal

Products/sites affected: (if applicable)

Describe the issue:

ComboBox, ContextualMenu, and TeachingBubble failed to merge system classes with props.calloutProps.className correctly.

Actual behavior:

When specifying a custom class via props.calloutProps.className, only the system classes are added to the Callout for ComboBox and ContextualMenu. With TeachingBubble, the user class is added but the system class is not.

Expected behavior:

When specifying a custom property via props.calloutProps.className, the class is added to the Callout element along with the system classes.

If applicable, please provide a codepen repro:

https://codepen.io/anon/pen/mpboVe