FormField is now a widget that can contain any type of field. Input no
longer has special code to handle form fields. Instead, there is a
helper widget InputFormField for using an Input inside a FormField.
Fixes https://github.com/flutter/flutter/issues/6097 and based on
feedback from the same.
* Add a maxLines parameter for multiline Input.
If maxLines is 1, it's a single line Input that scrolls horizontally.
Otherwise, overflowed text wraps and scrolls vertically, taking up at
most `maxLines`.
Also fixed scrolling behavior so that the Input scrolls ensuring the
cursor is always visible.
Fixes https://github.com/flutter/flutter/issues/6271
* oops
* comments
* import
* test and RO.update fix
* constant
* fix.caretRect
