Eliminate per-frame allocations in rendering pipeline, fix missing markup colors, add gradient backgrounds

- Replace LINQ chains with pooled lists and manual loops on all hot rendering paths
  (Renderer, RenderCoordinator, ScrollablePanelControl, WindowRenderer)
- Pool StringBuilders in TreeControl and ConsoleBuffer to avoid per-frame allocations
- Add StringBuilder overload to TerminalRawMode.WriteStdout using GetChunks()/Encoder.Convert()
  to eliminate large ToString() allocation on every frame
- Fix lock bug in ConsoleBuffer: lock(obj ?? new object()) created new lock each call
- Fix ScrollablePanelBuilder defaulting to Color.Black instead of null, blocking gradient inheritance
- Add 7 missing colors to ColorTable (gold1, gold3, coral, magenta1, dodgerblue2, springgreen2, chartreuse2)
- Add Color.Coral static property
- Add gradient background to MarkupSyntax demo window
- Wrap DataGrid right panel in ScrollablePanel
- Update MARKUP_SYNTAX.md with all registered color names
- Add DemoApp screenshot to EXAMPLES.md

Author: nickprotop

Examples/DemoApp/DemoWindows/DataGridWindow.cs

@@ -334,10 +334,17 @@ public static Window Create(ConsoleWindowSystem ws)
             .StickyBottom()
             .Build();
 
+        // Right panel: scrollable info + feature table
+        var infoPanel = Controls.ScrollablePanel()
+            .AddControl(infoMarkup)
+            .AddControl(featureTable)
+            .WithVerticalAlignment(VerticalAlignment.Fill)
+            .Build();
+
         // Layout: left = data grid, right = info panel
         var grid = Controls.HorizontalGrid()
             .Column(col => col.Add(dataGrid))
-            .Column(col => col.Width(38).Add(infoMarkup).Add(featureTable))
+            .Column(col => col.Width(38).Add(infoPanel))
             .WithSplitterAfter(0)
             .WithAlignment(HorizontalAlignment.Stretch)
             .WithVerticalAlignment(VerticalAlignment.Fill)

Examples/DemoApp/DemoWindows/MarkupSyntaxWindow.cs

@@ -1,6 +1,8 @@
 using SharpConsoleUI;
 using SharpConsoleUI.Builders;
 using SharpConsoleUI.Controls;
+using SharpConsoleUI.Helpers;
+using SharpConsoleUI.Rendering;
 
 namespace DemoApp.DemoWindows;
 
@@ -113,10 +115,16 @@ public static Window Create(ConsoleWindowSystem ws)
             .WithVerticalAlignment(SharpConsoleUI.Layout.VerticalAlignment.Fill)
             .Build();
 
+        var gradient = ColorGradient.FromColors(
+            new Color(45, 15, 70),
+            new Color(15, 50, 80),
+            new Color(60, 20, 75));
+
         return new WindowBuilder(ws)
             .WithTitle("Markup Syntax")
             .WithSize(85, 35)
             .Centered()
+            .WithBackgroundGradient(gradient, GradientDirection.Vertical)
             .AddControl(content)
             .OnKeyPressed((s, e) =>
             {

SharpConsoleUI/Builders/ScrollablePanelBuilder.cs

@@ -38,7 +38,7 @@ public sealed class ScrollablePanelBuilder : IControlBuilder<ScrollablePanelCont
 	private string? _name;
 	private object? _tag;
 	private StickyPosition _stickyPosition = StickyPosition.None;
-	private Color _backgroundColor = Color.Black;
+	private Color? _backgroundColor;
 	private Color _foregroundColor = Color.White;
 
 	/// <summary>
@@ -375,10 +375,15 @@ public ScrollablePanelControl Build()
 			Name = _name,
 			Tag = _tag,
 			StickyPosition = _stickyPosition,
-			BackgroundColor = _backgroundColor,
 			ForegroundColor = _foregroundColor
 		};
 
+		// Set background only if explicitly specified (null = transparent, inherits gradient)
+		if (_backgroundColor.HasValue)
+		{
+			control.BackgroundColor = _backgroundColor.Value;
+		}
+
 		// Add all children
 		foreach (var child in _children)
 		{

SharpConsoleUI/Color.cs

@@ -158,6 +158,7 @@ private Color(byte r, byte g, byte b, bool isDefault)
 		public static Color Chartreuse2 => new(118, 238, 0);
 		public static Color Gold1 => new(255, 215, 0);
 		public static Color Gold3 => new(205, 173, 0);
+		public static Color Coral => new(255, 127, 80);
 
 		#endregion
 

SharpConsoleUI/ColorTable.cs

@@ -115,6 +115,13 @@ internal static class ColorTable
 			["darkslategray3"] = Color.DarkSlateGray3,
 			["cyan1"] = Color.Cyan1,
 			["orange3"] = Color.Orange3,
+			["magenta1"] = Color.Magenta1,
+			["dodgerblue2"] = Color.DodgerBlue2,
+			["springgreen2"] = Color.SpringGreen2,
+			["chartreuse2"] = Color.Chartreuse2,
+			["gold1"] = Color.Gold1,
+			["gold3"] = Color.Gold3,
+			["coral"] = Color.Coral,
 
 			// Grey scale
 			["grey0"] = Color.Grey0,

SharpConsoleUI/Controls/ScrollablePanelControl/ScrollablePanelControl.Rendering.cs

@@ -100,7 +100,12 @@ public override void PaintDOM(CharacterBuffer buffer, LayoutRect bounds, LayoutR
 					fixedHeight += node.DesiredSize.Height;
 				}
 			}
-			int fillCount = paintSnapshot.Count(c => c.Visible && c.VerticalAlignment == VerticalAlignment.Fill);
+			int fillCount = 0;
+			foreach (var c in paintSnapshot)
+			{
+				if (c.Visible && c.VerticalAlignment == VerticalAlignment.Fill)
+					fillCount++;
+			}
 			int perFillHeight = (_viewportHeight > 0 && fillCount > 0)
 				? Math.Max(0, (_viewportHeight - fixedHeight) / fillCount) : _viewportHeight;
 
@@ -262,14 +267,19 @@ private int CalculateContentHeight(int viewportWidth, int maxHeight = 0)
 
 			List<IWindowControl> calcSnapshot;
 			lock (_childrenLock) { calcSnapshot = new List<IWindowControl>(_children); }
-			var visible = calcSnapshot.Where(c => c.Visible).ToList();
 
 			// Two-pass measurement: fixed children first, then Fill children get remaining space.
 			// Pass 1: measure non-Fill children to determine fixed height.
 			int fixedHeight = 0;
-			foreach (var child in visible)
+			int fillCount = 0;
+			foreach (var child in calcSnapshot)
 			{
-				if (child.VerticalAlignment == VerticalAlignment.Fill) continue;
+				if (!child.Visible) continue;
+				if (child.VerticalAlignment == VerticalAlignment.Fill)
+				{
+					fillCount++;
+					continue;
+				}
 				var childNode = LayoutNodeFactory.CreateSubtree(child);
 				childNode.IsVisible = true;
 				var constraints = new LayoutConstraints(1, availableWidth, 1, int.MaxValue);
@@ -278,15 +288,14 @@ private int CalculateContentHeight(int viewportWidth, int maxHeight = 0)
 			}
 
 			// Pass 2: measure Fill children with remaining space.
-			int fillCount = visible.Count(c => c.VerticalAlignment == VerticalAlignment.Fill);
 			int remainingHeight = (maxH < int.MaxValue) ? Math.Max(0, maxH - fixedHeight) : int.MaxValue;
 			int perFillHeight = (fillCount > 0 && remainingHeight < int.MaxValue)
 				? Math.Max(0, remainingHeight / fillCount) : int.MaxValue;
 
 			int totalHeight = fixedHeight;
-			foreach (var child in visible)
+			foreach (var child in calcSnapshot)
 			{
-				if (child.VerticalAlignment != VerticalAlignment.Fill) continue;
+				if (!child.Visible || child.VerticalAlignment != VerticalAlignment.Fill) continue;
 				var childNode = LayoutNodeFactory.CreateSubtree(child);
 				childNode.IsVisible = true;
 				var constraints = new LayoutConstraints(1, availableWidth, 1, perFillHeight);
@@ -301,8 +310,16 @@ private int CalculateContentWidth()
 		{
 			List<IWindowControl> snapshot;
 			lock (_childrenLock) { snapshot = new List<IWindowControl>(_children); }
-			var visibleChildren = snapshot.Where(c => c.Visible).ToList();
-			return visibleChildren.Any() ? visibleChildren.Max(c => c.GetLogicalContentSize().Width) : 0;
+			int maxWidth = 0;
+			foreach (var c in snapshot)
+			{
+				if (c.Visible)
+				{
+					int w = c.GetLogicalContentSize().Width;
+					if (w > maxWidth) maxWidth = w;
+				}
+			}
+			return maxWidth;
 		}
 
 		#endregion

SharpConsoleUI/Controls/TreeControl/TreeControl.Rendering.cs

@@ -8,7 +8,6 @@
 
 using SharpConsoleUI.Helpers;
 using SharpConsoleUI.Layout;
-using System.Text;
 
 namespace SharpConsoleUI.Controls
 {
@@ -22,25 +21,25 @@ private string BuildTreePrefix(int depth, bool[] ancestorIsLast, (string cross,
 			if (depth == 0)
 				return "";
 
-			StringBuilder prefix = new StringBuilder();
+			_prefixBuilder.Clear();
 
 			// For each ancestor level, draw a vertical continuation line if that ancestor
 			// still has siblings below it, or spaces if it was the last child.
 			for (int i = 0; i < depth - 1; i++)
 			{
-				prefix.Append(ancestorIsLast[i] ? " " : guides.vertical);
-				prefix.Append(_indent);
+				_prefixBuilder.Append(ancestorIsLast[i] ? " " : guides.vertical);
+				_prefixBuilder.Append(_indent);
 			}
 
 			// Add appropriate connector for the current node
 			bool isLast = ancestorIsLast[depth - 1];
 			string connector = isLast ? guides.corner : guides.tee;
 
-			prefix.Append(connector);
-			prefix.Append(guides.horizontal);
-			prefix.Append(" ");
+			_prefixBuilder.Append(connector);
+			_prefixBuilder.Append(guides.horizontal);
+			_prefixBuilder.Append(" ");
 
-			return prefix.ToString();
+			return _prefixBuilder.ToString();
 		}
 
 		private (string cross, string corner, string tee, string vertical, string horizontal) GetGuideChars()

SharpConsoleUI/Controls/TreeControl/TreeControl.cs

@@ -52,6 +52,9 @@ public partial class TreeControl : BaseControl, IInteractiveControl, IFocusableC
 		// Performance: Cache for expensive text measurement operations
 		private readonly TextMeasurementCache _textMeasurementCache = new(Parsing.MarkupParser.StripLength);
 
+		// Pooled StringBuilder to avoid per-node allocations during tree prefix building
+		private readonly System.Text.StringBuilder _prefixBuilder = new();
+
 		// Read-only helpers
 		private int CurrentSelectedIndex => _selectedIndex;
 		private int CurrentScrollOffset => _scrollOffset;