fix: drop zero-width runes with no base cell in MarkupParser

Previously, a zero-width rune at the start of a parsed line (e.g. stray
U+FEFF from Outlook HTML) was added as a standalone cell with a
zero-width character, making cells.Count one longer than the visual
width and misaligning every subsequent cell in MarkupControl rendering.
This desynchronized the terminal cursor from the buffer's bookkeeping
and broke rendering of the rest of the line.

Now matches CharacterBuffer.WriteString semantics: leading zero-width
runes with nothing to attach to are dropped. Tests updated to codify
the corrected contract.

Author: nickprotop

SharpConsoleUI.Tests/Parsing/MarkupParserUnicodeEdgeCaseTests.cs

@@ -410,35 +410,35 @@ public void Parse_BidiMarks_AttachAsCombiners()
 		#region Zero-Width at String Start
 
 		[Fact]
-		public void Parse_ZWJAtStart_CreatesOwnCell()
+		public void Parse_ZWJAtStart_IsDropped()
 		{
-			// ZWJ at start with no previous cell → falls through to own cell
+			// ZWJ at start with no previous cell to attach to is dropped.
 			var cells = MarkupParser.Parse("\u200DA", Color.White, Color.Black);
 
-			Assert.Equal(2, cells.Count);
-			Assert.Equal(new Rune('A'), cells[1].Character);
+			Assert.Single(cells);
+			Assert.Equal(new Rune('A'), cells[0].Character);
 		}
 
 		[Fact]
-		public void Parse_CombiningMarkAtStart_CreatesOwnCell()
+		public void Parse_CombiningMarkAtStart_IsDropped()
 		{
-			// U+0301 at start with no base → creates own cell
+			// U+0301 at start with no base cell to attach to is dropped.
 			var cells = MarkupParser.Parse("\u0301A", Color.White, Color.Black);
 
-			Assert.Equal(2, cells.Count);
-			Assert.Equal(new Rune('A'), cells[1].Character);
+			Assert.Single(cells);
+			Assert.Equal(new Rune('A'), cells[0].Character);
 		}
 
 		[Fact]
-		public void Parse_MultipleCombinersAtStart_EachCreatesCell()
+		public void Parse_MultipleCombinersAtStart_AllDropped()
 		{
-			// Two zero-width at start: first creates cell, second attaches to first
+			// Leading zero-width runes with no base cell are all dropped;
+			// the first real base cell starts fresh without their combiners.
 			var cells = MarkupParser.Parse("\u0300\u0301A", Color.White, Color.Black);
 
-			// First U+0300 creates own cell (no previous), U+0301 attaches to it
-			Assert.Equal(2, cells.Count);
-			Assert.NotNull(cells[0].Combiners); // U+0301 attached
-			Assert.Equal(new Rune('A'), cells[1].Character);
+			Assert.Single(cells);
+			Assert.Equal(new Rune('A'), cells[0].Character);
+			Assert.Null(cells[0].Combiners);
 		}
 
 		#endregion

SharpConsoleUI.Tests/Parsing/MarkupParserWideCharTests.cs

@@ -376,14 +376,16 @@ public void Parse_MultipleCombinersOnOneCell()
 		}
 
 		[Fact]
-		public void Parse_ZeroWidthAtStartOfString_CreatesOwnCell()
+		public void Parse_ZeroWidthAtStartOfString_IsDropped()
 		{
-			// If zero-width char appears first with no previous cell to attach to,
-			// it falls through to create its own cell (defensive behavior)
+			// If a zero-width char appears first with no previous cell to attach to,
+			// it is dropped. Keeping it as a standalone cell would desynchronize
+			// cell-count from visual width and misalign every subsequent cell when
+			// painted (e.g. Outlook's stray U+FEFF breaking rendering of the line).
 			var cells = MarkupParser.Parse("\uFE0FA", Color.White, Color.Black);
 
-			Assert.Equal(2, cells.Count);
-			Assert.Equal(new Rune('A'), cells[1].Character);
+			Assert.Single(cells);
+			Assert.Equal(new Rune('A'), cells[0].Character);
 		}
 
 		#endregion

SharpConsoleUI/Parsing/MarkupParser.cs

@@ -156,25 +156,32 @@ public static List<Cell> Parse(string markup, Color defaultFg, Color defaultBg)
 					if (Rune.TryGetRuneAt(markup, i, out var rune))
 					{
 						int runeWidth = GetRuneWidth(rune);
-						if (runeWidth == 0 && cells.Count > 0)
+						if (runeWidth == 0)
 						{
-							var lastIdx = cells.Count - 1;
-							// Skip past continuation cells to attach to the base cell
-							if (cells[lastIdx].IsWideContinuation && lastIdx > 0)
-								lastIdx--;
-							var lastCell = cells[lastIdx];
-							// VS16 widens certain emoji from 1→2 columns
-							if (IsVS16(rune) && IsVs16Widened(lastCell.Character) && !IsWideRune(lastCell.Character))
+							if (cells.Count > 0)
 							{
-								lastCell.AppendCombiner(rune);
-								cells[lastIdx] = lastCell;
-								cells.Add(new Cell(' ', currentFg, currentBg, currentDec) { IsWideContinuation = true });
-							}
-							else
-							{
-								lastCell.AppendCombiner(rune);
-								cells[lastIdx] = lastCell;
+								var lastIdx = cells.Count - 1;
+								// Skip past continuation cells to attach to the base cell
+								if (cells[lastIdx].IsWideContinuation && lastIdx > 0)
+									lastIdx--;
+								var lastCell = cells[lastIdx];
+								// VS16 widens certain emoji from 1→2 columns
+								if (IsVS16(rune) && IsVs16Widened(lastCell.Character) && !IsWideRune(lastCell.Character))
+								{
+									lastCell.AppendCombiner(rune);
+									cells[lastIdx] = lastCell;
+									cells.Add(new Cell(' ', currentFg, currentBg, currentDec) { IsWideContinuation = true });
+								}
+								else
+								{
+									lastCell.AppendCombiner(rune);
+									cells[lastIdx] = lastCell;
+								}
 							}
+							// else: zero-width rune with no preceding base cell — drop it.
+							// Creating a standalone cell for a zero-width rune desynchronizes
+							// cell-count from visual width and misaligns every subsequent cell
+							// (e.g. the FEFF-at-start-of-line rendering bug with Outlook HTML).
 						}
 						else
 						{