This needs to be fixed and should be fixed using the modern View.ContentArea/Viewport + View.Horizontal/VerticalScrollBar.

AutoShow on vertical should be on by default. Horizontal should track WordWrap.

There are two approaches:

1. Upgrade TextView to utilize the virtual content scrolling now built into View via Viewport, ViewportSettings, and SetContentSize().
2. Leave the existing scroll logic in place, and program View.HorizontalScrollBar/VerticalScrollBar manually.

The former is a far better solution and should not be too hard to accomplish.