One of nice little features I like about the Intellij Idea IDE is the full display of a truncated tree item on hover. You can see an example of this in the attached image where I hover the mouse over the FavIconHandler class and it's full name is displayed.

While a user could implement such a feature using the basic primitives provided by JavaFX today by creating their a special CellFactory, I think the feature is useful enough that it may warrant addition to the base JavaFX platform.

Examples of candidates for overflow display on mouseover are default listcells, treecells, comboboxcells and tableviewcells.