Steps to reproduce:
- Go to a template page (e.g. https://www.mediawiki.org/w/index.php?title=Template:Extension&action=edit) and click edit
- Click the templatedata button
- Click on a parameter
- Type something in label and/or description
The typed text should appear in the text field
The input will be cleared immediatly, so it's impossible to insert text there.
It's working on wikipedia's, so it's seems to be a problem in 1.25wmf21