The following GIF shows multiple bugs:
1. The TextArea's Material.accent property is not respected inside the Flickable.
2. The placeholderText is not part of the decorator, so it scrolls along the content.
3. The CursorDelegate is shown beyond the TextArea's limits even when clipped.

EDIT: There's is actually a fourth bug that I explain more carefully in the second comment:
4. The placeholderText is cropped at half when the TextArea is clipped. This also applies to TextField.

Source code (run with qml tool):

import QtQuick
import QtQuick.Controls.Material

ApplicationWindow {
    width: 400
    height: 400
    visible: true

    Flickable {
        anchors.fill: parent
        anchors.margins: 100

        TextArea.flickable: TextArea {
            placeholderText: "Type something..."
            text: "a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\nm\nn"
        }
    }
}