Fixed GetOffsetFromMousePositionFirstTextLineOnly() not considering the horizontal scroll offset.

This fixes creating a rectangular selection with the mouse while scrollOffset.X > 0.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Editing/SelectionMouseHandler.cs

@@ -527,7 +527,7 @@ namespace ICSharpCode.AvalonEdit.Editing
 				pos.Y = textView.DocumentHeight - ExtensionMethods.Epsilon;
 			VisualLine line = textView.GetVisualLineFromVisualTop(pos.Y);
 			if (line != null) {
-				visualColumn = line.GetVisualColumn(line.TextLines.First(), positionRelativeToTextView.X, textArea.Selection.EnableVirtualSpace);
+				visualColumn = line.GetVisualColumn(line.TextLines.First(), pos.X, textArea.Selection.EnableVirtualSpace);
 				return line.GetRelativeOffset(visualColumn) + line.FirstDocumentLine.Offset;
 			}
 			return -1;