Vaadin Bot

Implement drag'n'drop for TwinColSelect

3

**Originally by @jouni** ___ TwinColSelect should allow user to move items from one list to another with drag'n'drop. ___ Imported from https://dev.vaadin.com/ issue #4389

Add support for custom view providers

5

**Originally by _jaroslaw_przygodzki_** --- It's not immediately obvious (or at least not explicitly stated anywhere in the documentation, as far as I can tell) but in order to ensure correct...