Changed highlighting engine to avoid 'merging' colors when both the <Span> and <Begin> tags have a color specified.

Instead, we create a nested HighlightingSection for the <Begin>-color within the section for the <Span>-color.
This fixes customization of such colors.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/DocumentHighlighter.cs

@@ -282,11 +282,14 @@ namespace ICSharpCode.AvalonEdit.Highlighting
 				Debug.Assert(position == firstMatch.Index);
 				
 				if (firstMatch == endSpanMatch) {
-					PopColor(); // pop SpanColor
 					HighlightingSpan poppedSpan = spanStack.Peek();
+					if (!poppedSpan.SpanColorIncludesEnd)
+						PopColor(); // pop SpanColor
 					PushColor(poppedSpan.EndColor);
 					position = firstMatch.Index + firstMatch.Length;
 					PopColor(); // pop EndColor
+					if (poppedSpan.SpanColorIncludesEnd)
+						PopColor(); // pop SpanColor
 					spanStack = spanStack.Pop();
 					currentRuleSet = this.CurrentRuleSet;
 					//FreeMatchArray(matches);
@@ -312,9 +315,12 @@ namespace ICSharpCode.AvalonEdit.Highlighting
 					currentRuleSet = this.CurrentRuleSet;
 					storedMatchArrays.Push(matches);
 					matches = AllocateMatchArray(currentRuleSet.Spans.Count);
+					if (newSpan.SpanColorIncludesStart)
+						PushColor(newSpan.SpanColor);
 					PushColor(newSpan.StartColor);
 					position = firstMatch.Index + firstMatch.Length;
 					PopColor();
+					if (!newSpan.SpanColorIncludesStart)
 						PushColor(newSpan.SpanColor);
 				}
 				endSpanMatch = null;

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/HighlightingSpan.cs

@@ -43,6 +43,18 @@ namespace ICSharpCode.AvalonEdit.Highlighting
 		/// </summary>
 		public HighlightingColor EndColor { get; set; }
 		
+		/// <summary>
+		/// Gets/Sets whether the span color includes the start.
+		/// The default is <c>false</c>.
+		/// </summary>
+		public bool SpanColorIncludesStart { get; set; }
+		
+		/// <summary>
+		/// Gets/Sets whether the span color includes the end.
+		/// The default is <c>false</c>.
+		/// </summary>
+		public bool SpanColorIncludesEnd { get; set; }
+		
 		/// <inheritdoc/>
 		public override string ToString()
 		{

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Highlighting/Xshd/XmlHighlightingDefinition.cs

@@ -317,22 +317,11 @@ namespace ICSharpCode.AvalonEdit.Highlighting.Xshd
 					StartExpression = CreateRegex(span, span.BeginRegex, span.BeginRegexType),
 					EndExpression = CreateRegex(span, endRegex, span.EndRegexType),
 					RuleSet = GetRuleSet(span, span.RuleSetReference),
-					StartColor = MergeColor(wholeSpanColor, GetColor(span, span.BeginColorReference)),
+					StartColor = GetColor(span, span.BeginColorReference),
 					SpanColor = wholeSpanColor,
-					EndColor = MergeColor(wholeSpanColor, GetColor(span, span.EndColorReference)),
-				};
-			}
-			
-			static HighlightingColor MergeColor(HighlightingColor baseColor, HighlightingColor newColor)
-			{
-				if (baseColor == null)
-					return newColor;
-				if (newColor == null)
-					return baseColor;
-				return new HighlightingColor {
-					Foreground = newColor.Foreground ?? baseColor.Foreground,
-					FontWeight = newColor.FontWeight ?? baseColor.FontWeight,
-					FontStyle = newColor.FontStyle ?? baseColor.FontStyle,
+					EndColor = GetColor(span, span.EndColorReference),
+					SpanColorIncludesStart = true,
+					SpanColorIncludesEnd = true
 				};
 			}