You wrote
We had already some proposal for feature spec:
http://anonsvn.jboss.org/repos/richfaces/management/design/Split%20Panel%20component/
It uses the first approach from the list above. The feature used to be proposed to be implemented in RichFaces 3.3.0, but was not approved
This feature was not approved for implementation in 3.3.1-3.3.2. According to votes stabilization work is preferable for branch. So we shifter complex approaches to 4.0.x version.
As Ilya said this was a micro release, and we needed to keep complexity down. This type of layout is in the works for 4.0, but will also be discussed in relation to the other big ticket items.
-Jay