Fix UI freeze when drag-selecting past the right edge

[mgarstenauer]

Fixes #606

Problem

The UI will freeze when dragging the selection past the right edge of the text editor. See details in #606.

Cause

The expected behavior is that the scrolling stops when the end of the scroll bar is reached. However, in this case Caret.BringIntoView(5.0) is called, which adds an artificial border around the caret. When this border lies outside of the range of the scroll viewer then TextView.MakeVisible sets an invalid, out-of-range scroll offset. This triggers another ScrollOffsetChanged event with the invalid scroll offset. This causes an endless loop, the drag-selecting/scrolling never stops.

Solution

In TextView.MakeVisible clamp the scroll offset to the allowed range.

[mgarstenauer]

In most cases only caret.BringCaretToView() is called, which doesn't guarantee that the caret is visible.

For example (tested in AvaloniaEdit.Demo): On a long line when I press End, the caret jumps to the end of the line. The end of the line is visible, but the caret is just slightly out of view.

⇒ I suggest to make BringCaretToView(5.0) the default.