chore: import upstream snapshot with attribution

dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs

@@ -0,0 +1,88 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Harness.ConsoleReactiveFramework;
+
+namespace Harness.ConsoleReactiveComponents;
+
+/// <summary>
+/// Props for <see cref="TextScrollPanel"/>.
+/// </summary>
+public record TextScrollPanelProps : ConsoleReactiveProps
+{
+    /// <summary>Gets the items to render in the scroll panel. Each item is a pre-rendered
+    /// console string (may include ANSI escape sequences and newlines).</summary>
+    public IReadOnlyList<string> Items { get; init; } = [];
+}
+
+/// <summary>
+/// State for <see cref="TextScrollPanel"/>.
+/// </summary>
+public record TextScrollPanelState : ConsoleReactiveState;
+
+/// <summary>
+/// A component that renders pre-rendered string items within a scroll area.
+/// The last rendered item is considered dynamic and will be re-rendered on each call.
+/// All prior items are considered finalized and are not re-rendered.
+/// Use <see cref="Invalidate"/> to force a full re-render.
+/// </summary>
+public class TextScrollPanel : ConsoleReactiveComponent<TextScrollPanelProps, TextScrollPanelState>
+{
+    private int _renderedCount;
+    private int _lastItemOffsetFromBottom;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TextScrollPanel"/> class.
+    /// </summary>
+    public TextScrollPanel()
+    {
+        this.State = new TextScrollPanelState();
+    }
+
+    /// <inheritdoc />
+    public override void Invalidate()
+    {
+        this._renderedCount = 0;
+        this._lastItemOffsetFromBottom = 0;
+        base.Invalidate();
+    }
+
+    /// <inheritdoc />
+    public override void RenderCore(TextScrollPanelProps props, TextScrollPanelState state)
+    {
+        if (props.Items.Count == 0)
+        {
+            return;
+        }
+
+        int bottomRow = props.Y + props.Height - 1;
+
+        // Determine the first item to render. If we previously rendered items,
+        // re-render the last one (it may have changed/grown) from its stored position.
+        int startIndex = this._renderedCount > 0 ? this._renderedCount - 1 : 0;
+
+        if (this._renderedCount > 0 && this._lastItemOffsetFromBottom > 0)
+        {
+            // Reposition cursor to where the last rendered item began
+            Console.Write(AnsiEscapes.MoveCursor(bottomRow - this._lastItemOffsetFromBottom, props.X));
+        }
+        else
+        {
+            // First render — position at the bottom of the scroll area
+            Console.Write(AnsiEscapes.MoveCursor(bottomRow, props.X));
+        }
+
+        // Render from startIndex onwards
+        for (int i = startIndex; i < props.Items.Count; i++)
+        {
+            Console.Write(props.Items[i]);
+        }
+
+        // Calculate the offset from bottom for the start of the new last item,
+        // accounting for terminal line wrapping at the available width.
+        int lastItemLines = AnsiEscapes.CountPhysicalLines(props.Items[^1], props.Width);
+        this._lastItemOffsetFromBottom = lastItemLines > 0 ? lastItemLines - 1 : 0;
+
+        // Update rendered count
+        this._renderedCount = props.Items.Count;
+    }
+}