Steps to replicate the issue:

• Edit a template on enwiki in source mode
• Click the "Publish changes..." button
• Without putting anything into the "Preview page with this template" text box, click "Show preview"
• Dismiss the warning that says "The title you specified for previewing is invalid."
• Click "Show changes" or "Show preview" buttons. The back button will be disabled. You have to press the escape button to exit out of the dialog and press the "Publish changes..." button again.

What happens?:

The back button is disabled.

What should have happened instead?:

The back button should not be disabled.