Material-ui: StepContent doesn't display when componentized outside of Stepper

[jonrhall]

When you use the Stepper component in vertical orientation, you use the StepContent component as a way to display content for the active step. When I tried to refactor StepContent into its own React component, Material-UI throws a warning in the console saying Warning: Material-UI: <StepContent /> is only designed for use with the vertical stepper. Additionally, the StepContent component itself doesn't display at all.

• [x] This is a v1.x issue.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

Refactoring StepContent into its own custom React component shouldn't break the StepContent component being displayed in the Stepper component with vertical orientation.

Current Behavior

Material-UI throws a warning in the console saying Warning: Material-UI: <StepContent /> is only designed for use with the vertical stepper. Additionally, the StepContent component itself doesn't display at all.

Steps to Reproduce

StepContent working (embedded inside Stepper): https://codesandbox.io/s/l573xxp0ll
StepContent broken (inside a custom React.Component): https://codesandbox.io/s/624xo0o5kw

1. Open the two links
2. Compare the results. Only basic changes were made to show the problem.

Context

My Stepper component was becoming rather verbose, and I wanted to refactor just the active step content display into its own file to encapsulate my logic better.

Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | v1.1.0 |
| React | 16.4.0 |
| browser | Chrome |

[mbrookes]

You need to proxy the props down to StepContent: https://codesandbox.io/s/8nxrv89w3l

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