Steps to Reproduce

run the simplified attached example tree-remains-empty-zk4804.zul
or this zkfiddle

scroll to about half the height (around item 4000)

click "remove TreeModel" (sets model to null)
click "add TreeModel"

Current Result

tree fails to render contents correctly (not always, seems timing dependent)
the tree remains at the current scroll position (even after clearing the model)

Expected Result

when the model is set to null, the scroll position should to reset to top

Debug Information

even calling tree.invalidate() after clearing and adding a tree model fixes the issue.

Workaround

call the private method tree.resetPosition(false) via reflection