The JavaFX collections framework offers the `FXCollections.concat(ObservableList...)` method, which creates a new modifiable list that contains a concatenated snapshot of the source lists, backed by an `ArrayList`. However, when the source lists are changed, the concatenated list is not updated.

A new method adds this missing capability: `FXCollections.concatenatedObservableList(ObservableList...)`, which creates an unmodifiable collection view onto the source lists that represents the concatenation of all source elements. When any of the source lists are changed, the change is also reflected in the concatenated view.