ScrollBar in Flickable misbehaving

[ocgltd]

I have created a TextArea inside a flickable, and when I add lots of newlines the area scrolls as expected. However, although a scrollbar appears when I type more lines than the available height, if I try to drag the scrollbar it resets to fully colored (nothing to drag) and just generally misbehaves. A related problem, if i DOWNARROW to the bottom line then my visible text jumps to the top, even though the cursor remains at the bottom. (And DOWNARROW key calls a scrollbar method).

What is wrong with my scrollbar?

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12

Window {
    id: box
    width: 640
    height: 180
    visible: true
    title: qsTr("ScrollBar Mystery")

    Flickable {
        id: inputWrapper
        anchors.fill: parent

        ScrollBar.vertical: ScrollBar {
            id: scrollBar
            policy: ScrollBar.AlwaysOn
            anchors.left: box.right
        }
        Keys.onUpPressed: scrollBar.decrease()
        Keys.onDownPressed: scrollBar.increase()

        clip: true
         flickableDirection: Flickable.VerticalFlick
        function ensureVisible(r)
        {
            if (contentX >= r.x)
                contentX = r.x;
            else if (contentX+width <= r.x+r.width)
                contentX = r.x+r.width-width;
            if (contentY >= r.y)
                contentY = r.y;
            else if (contentY+height <= r.y+r.height)
                contentY = r.y+r.height-height;
        }
        TextEdit {
            id: input
            anchors.fill: parent
            text: ""
            focus: true
            wrapMode: TextEdit.Wrap
            onCursorRectangleChanged: inputWrapper.ensureVisible(cursorRectangle)
        }  // TextEdit
    }  // Flickable
}  // Window

[J.Hilk]

@ocgltd

add
contentHeight: input.contentHeight
to your Flickable