steps to reproduce:

1. run the attached example: mvn jetty:run
2. launch in browser: http://localhost:8080/treeupdates/test3.zul (page will start high frequent updates on the tree, changing values and css classes)
3. open several nodes
4. wait ... , after some time the nodes will start to disappear/reappear and eventually disappear completely

5. with the "toggle updated" button the server side updates can be stopped(and started), so the tree remains unchanged on the client side
6. pressing "invalidate" will refresh the tree showing it is a client side problem, as the tree state can be recovered from the server

as defined in the pom the issue can be reproduced with latest freshly 6.5.3.FL.20130424

running the sample with 6.5.1.1 will not show this problem tree nodes will remain stable
running the sample with 6.5.2 will result in browser freeze (fixed with ZK-1726)