After clicking the 'Defaults' button in the customized world settings UI, the options become unresponsive to all mouse and keyboard input.
- Open the create world page
- Select world type Customized
- Open the 'Customize World Settings' UI
- Change some settings or click 'Randomize'
- Click 'Defaults'
- Try to change the settings manually
Backing out of the customize settings page and reopening it will make the options responsive again.