This is a regular patch release, which includes some important bug fixes but no feature enhancements over the previous release. It is fully backwards-compatible to qooxdoo 2.1.x. Nothing needs to be changed in the JavaScript code of your existing apps if they are based on the previous 2.1.1 or 2.1 releases.
See the corresponding bugzilla query for the issues resolved for qooxdoo 2.1.2.