VisualLine.GetVisualColumnFloor bugfix: for positions after the end of the line; return the line's end column.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/VisualLine.cs

@@ -53,7 +53,7 @@ namespace ICSharpCode.AvalonEdit.Rendering
 		/// Gets the start offset of the VisualLine inside the document.
 		/// This is equivalent to <c>FirstDocumentLine.Offset</c>.
 		/// </summary>
-		public int StartOffset { 
+		public int StartOffset {
 			get {
 				return FirstDocumentLine.Offset;
 			}
@@ -323,6 +323,12 @@ namespace ICSharpCode.AvalonEdit.Rendering
 		public int GetVisualColumnFloor(Point point)
 		{
 			TextLine textLine = GetTextLineByVisualYPosition(point.Y);
+			if (point.X > textLine.WidthIncludingTrailingWhitespace) {
+				// GetCharacterHitFromDistance returns a hit with FirstCharacterIndex=last character inline
+				// and TrailingLength=1 when clicking behind the line, so the floor function needs to handle this case
+				// specially end return the line's end column instead.
+				return GetTextLineVisualStartColumn(textLine) + textLine.Length;
+			}
 			CharacterHit ch = textLine.GetCharacterHitFromDistance(point.X);
 			return ch.FirstCharacterIndex;
 		}