Fixed #600: Incorrect TextBox selection coloring for non-left aligned text.

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -106,6 +106,11 @@ namespace Avalonia.Controls.Presenters
             {
                 var start = Math.Min(selectionStart, selectionEnd);
                 var length = Math.Max(selectionStart, selectionEnd) - start;
+
+                // issue #600: set constaint before any FormattedText manipulation
+                //             see base.Render(...) implementation
+                FormattedText.Constraint = Bounds.Size;
+
                 var rects = FormattedText.HitTestTextRange(start, length);
 
                 if (_highlightBrush == null)