Bug when scrolling text and trying to type something in a TextView

[luizfernandonb]

Describe the bug
The bug occurs when you scroll to several lines beyond the mouse cursor and soon after you try to write something.

Extra: In a TextView.

To Reproduce
Steps to reproduce the behavior:

1. Open a file in a TextView that has several lines and that you can scroll using the mouse wheel.
2. Scroll until you don't see the cursor, which isn't much.
3. Try typing something.
4. See error

Expected behavior
What you typed in the first line of the console, because in theory, the cursor is to follow the mouse scroll when the text is scrolled, or when you type the text, it goes to the line that the cursor was on.

Screenshots
Open any text file:

Scroll down the mouse to a line where it disappears:

Tried typing anything on the keyboard:

[tznind]

Thanks for reporting this. I can confirm that I am able to reproduce and here is a unit test for this issue:

[Fact]
[AutoInitShutdown]
public void ScrollDownTillCaretOffscreen_ThenType()
{
	var tv = new TextView {
		Width = 10,
		Height = 5
	};

	// add 100 lines of wide text to view
	for (int i = 0; i < 100; i++)
		tv.Text += new string ('x', 100) + Environment.NewLine;

	Assert.Equal (0, tv.CursorPosition.Y);
	tv.ScrollTo (50);
	Assert.Equal (0, tv.CursorPosition.Y);

	tv.ProcessKey (new KeyEvent (Key.p, new KeyModifiers ()));
}

Error is currently:

Message: 
System.ArgumentException : Height must be greater or equal to 0.

  Stack Trace: 
Rect.set_Height(Int32 value) line 51
Rect.ctor(Int32 x, Int32 y, Int32 width, Int32 height) line 251
TextView.Insert(Rune rune) line 2387
TextView.InsertText(KeyEvent kb) line 3454
TextView.ProcessKey(KeyEvent kb) line 2609
TextViewTests.ScrollDownTillCaretOffscreen_ThenType() line 5714

Normally @BDisp handles TextView changes as he is most expert with the view