How can I make a TextView automatically scroll as I add more lines of text?

From your Code, two steps have to do:

Step 1

Although you code in Xamarin Android, but as in Java in the xxxActivity.java for terminalOutput invoke must be like this

TextView outputText = (TextView) findViewById(R.id.terminalOutput);
outputText.setMovementMethod(new ScrollingMovementMethod());

Method setMovementMethod() by parameter ScrollingMovementMethod() is the gimmick.

Step 2

And in the layout as in Java-Android also in activity_xxx.xml for the TextView Declaration as above must have to add this

android:gravity="bottom"

---

When you add new line into the outputText like this:

outputText.append("\n"+"New text line.");

It will scroll to the last line automatically, and these all the magic for your need.

As per answer here Making TextView Scrollable in Android

You don't need to use a ScrollView actually.

Just set the

android:maxLines = "AN_INTEGER"

android:scrollbars = "vertical" properties of your TextView in your layout's xml file.

Then use:

yourTextView.setMovementMethod(new ScrollingMovementMethod());

in your code.

That will work..