From 0099a6e2fa4c626629d0a6aaf53c085ead52490d Mon Sep 17 00:00:00 2001
From: westey <164392973+westey-m@users.noreply.github.com>
Date: Mon, 25 May 2026 10:25:58 +0100
Subject: [PATCH] .NET: HarnessConsole: Improve rendering perf / reduce
 flickering (#6014)

* HarnessConsole: Improve rendering perf / reduce flickering

* Address PR comments
---
 .../ListSelection.cs                          |  6 +-
 .../TextScrollPanel.cs                        | 76 +++++++++++++++----
 .../ConsoleReactiveMemo.cs                    | 36 +++++++++
 .../HarnessAppComponent.cs                    | 41 ++--------
 4 files changed, 108 insertions(+), 51 deletions(-)
 create mode 100644 dotnet/samples/02-agents/Harness/ConsoleReactiveFramework/ConsoleReactiveMemo.cs

diff --git a/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/ListSelection.cs b/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/ListSelection.cs
index eedc76a145..cdb13c6930 100644
--- a/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/ListSelection.cs
+++ b/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/ListSelection.cs
@@ -40,8 +40,8 @@ public class ListSelection : ConsoleReactiveComponent<ListSelectionProps, Consol
             foreach (string line in props.Title.Split('\n'))
             {
                 Console.Write(AnsiEscapes.MoveCursor(props.Y + row, props.X));
-                Console.Write(AnsiEscapes.EraseEntireLine);
                 Console.Write(line);
+                Console.Write(AnsiEscapes.EraseToEndOfLine);
                 row++;
             }
         }
@@ -52,7 +52,6 @@ public class ListSelection : ConsoleReactiveComponent<ListSelectionProps, Consol
         for (int i = 0; i < totalItems; i++)
         {
             Console.Write(AnsiEscapes.MoveCursor(props.Y + row, props.X));
-            Console.Write(AnsiEscapes.EraseEntireLine);
 
             bool isSelected = i == props.SelectedIndex;
             bool isCustomTextOption = props.CustomTextPlaceholder != null && i == props.Items.Count;
@@ -72,6 +71,7 @@ public class ListSelection : ConsoleReactiveComponent<ListSelectionProps, Consol
                 }
 
                 Console.Write(props.Items[i]);
+                Console.Write(AnsiEscapes.EraseToEndOfLine);
 
                 if (isSelected)
                 {
@@ -101,6 +101,7 @@ public class ListSelection : ConsoleReactiveComponent<ListSelectionProps, Consol
             }
 
             Console.Write(props.CustomText);
+            Console.Write(AnsiEscapes.EraseToEndOfLine);
 
             if (isSelected)
             {
@@ -121,6 +122,7 @@ public class ListSelection : ConsoleReactiveComponent<ListSelectionProps, Consol
 
             Console.Write(" ");
             Console.Write(props.CustomTextPlaceholder);
+            Console.Write(AnsiEscapes.EraseToEndOfLine);
             Console.Write(AnsiEscapes.ResetAttributes);
         }
     }
diff --git a/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs b/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs
index 15147b0fd0..3d86a400b3 100644
--- a/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs
+++ b/dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs
@@ -17,16 +17,19 @@ public record TextScrollPanelProps : ConsoleReactiveProps
 /// <summary>
 /// State for <see cref="TextScrollPanel"/>.
 /// </summary>
-/// <param name="RenderedCount">The number of items already rendered.</param>
-public record TextScrollPanelState(int RenderedCount = 0) : ConsoleReactiveState;
+public record TextScrollPanelState : ConsoleReactiveState;
 
 /// <summary>
 /// A component that renders pre-rendered string items within a scroll area.
-/// All items are considered finalized — only new items since the last render are output.
-/// Use <see cref="Reset"/> to force a full re-render.
+/// 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>
@@ -35,12 +38,12 @@ public class TextScrollPanel : ConsoleReactiveComponent<TextScrollPanelProps, Te
         this.State = new TextScrollPanelState();
     }
 
-    /// <summary>
-    /// Resets the panel so all items will be re-rendered on the next Render call.
-    /// </summary>
-    public void Reset()
+    /// <inheritdoc />
+    public override void Invalidate()
     {
-        this.State = new TextScrollPanelState();
+        this._renderedCount = 0;
+        this._lastItemOffsetFromBottom = 0;
+        base.Invalidate();
     }
 
     /// <inheritdoc />
@@ -51,16 +54,59 @@ public class TextScrollPanel : ConsoleReactiveComponent<TextScrollPanelProps, Te
             return;
         }
 
-        // Move cursor to the bottom of the scroll area
-        Console.Write(AnsiEscapes.MoveCursor(props.Y + props.Height - 1, props.X));
+        int bottomRow = props.Y + props.Height - 1;
 
-        // Output only new items since last rendered
-        for (int i = state.RenderedCount; i < props.Items.Count; i++)
+        // 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]);
         }
 
-        // Update state to track what we've rendered
-        this.State = new TextScrollPanelState(props.Items.Count);
+        // Calculate the offset from bottom for the start of the new last item
+        int lastItemLines = CountLines(props.Items[^1]);
+        this._lastItemOffsetFromBottom = lastItemLines > 0 ? lastItemLines - 1 : 0;
+
+        // Update rendered count
+        this._renderedCount = props.Items.Count;
+    }
+
+    private static int CountLines(string text)
+    {
+        if (string.IsNullOrEmpty(text))
+        {
+            return 0;
+        }
+
+        int count = 1;
+        for (int i = 0; i < text.Length; i++)
+        {
+            if (text[i] == '\n')
+            {
+                count++;
+            }
+        }
+
+        // If text ends with a newline, don't count the trailing empty line
+        if (text[text.Length - 1] == '\n')
+        {
+            count--;
+        }
+
+        return count;
     }
 }
diff --git a/dotnet/samples/02-agents/Harness/ConsoleReactiveFramework/ConsoleReactiveMemo.cs b/dotnet/samples/02-agents/Harness/ConsoleReactiveFramework/ConsoleReactiveMemo.cs
new file mode 100644
index 0000000000..1e278cd6e7
--- /dev/null
+++ b/dotnet/samples/02-agents/Harness/ConsoleReactiveFramework/ConsoleReactiveMemo.cs
@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Harness.ConsoleReactiveFramework;
+
+/// <summary>
+/// Caches the result of a mapping function and only recomputes when the input changes.
+/// </summary>
+/// <typeparam name="TInput">The type of the input value.</typeparam>
+/// <typeparam name="TOutput">The type of the mapped output value.</typeparam>
+public class ConsoleReactiveMemo<TInput, TOutput>
+{
+    private TInput? _previousInput;
+    private TOutput? _cachedOutput;
+    private bool _hasValue;
+
+    /// <summary>
+    /// Returns the cached output if <paramref name="input"/> equals the previously stored input;
+    /// otherwise invokes <paramref name="mapper"/> to compute and cache a new output.
+    /// </summary>
+    /// <param name="input">The current input value.</param>
+    /// <param name="mapper">A function that maps the input to an output value.</param>
+    /// <returns>The cached or newly computed output.</returns>
+    public TOutput Map(TInput input, Func<TInput, TOutput> mapper)
+    {
+        ArgumentNullException.ThrowIfNull(mapper);
+
+        if (!this._hasValue || !EqualityComparer<TInput>.Default.Equals(input, this._previousInput))
+        {
+            this._previousInput = input;
+            this._cachedOutput = mapper(input);
+            this._hasValue = true;
+        }
+
+        return this._cachedOutput!;
+    }
+}
diff --git a/dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessAppComponent.cs b/dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessAppComponent.cs
index 9521715467..d100c9d081 100644
--- a/dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessAppComponent.cs
+++ b/dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessAppComponent.cs
@@ -19,7 +19,6 @@ public class HarnessAppComponent : ConsoleReactiveComponent<ConsoleReactiveProps
     private readonly ListSelection _listSelection = new();
     private readonly TextInput _textInput = new();
     private readonly TextScrollPanel _textScrollPanel = new();
-    private readonly TextPanel _textPanel = new();
     private readonly TextPanel _queuedPanel = new();
     private readonly AgentStatus _agentStatus = new();
     private readonly AgentModeAndHelp _modeAndHelp = new();
@@ -341,16 +340,6 @@ public class HarnessAppComponent : ConsoleReactiveComponent<ConsoleReactiveProps
             return;
         }
 
-        // Determine the text panel height for the last scroll item
-        IReadOnlyList<string> lastItems = state.ScrollAreaContentItems.Count > 0
-            ? [state.ScrollAreaContentItems[^1]]
-            : [];
-        int textPanelHeight = TextPanel.CalculateHeight(lastItems);
-        if (textPanelHeight > 0)
-        {
-            textPanelHeight++; // Extra line for spacing between text panel and rule
-        }
-
         // Calculate queued items panel height
         int queuedPanelHeight = TextPanel.CalculateHeight(state.QueuedItems);
 
@@ -444,7 +433,7 @@ public class HarnessAppComponent : ConsoleReactiveComponent<ConsoleReactiveProps
         int modeAndHelpHeight = showStatusAndHelp ? AgentModeAndHelp.CalculateHeight(modeAndHelpProps) : 0;
 
         int ruleHeight = TopBottomRule.CalculateHeight(ruleProps);
-        int nonScrollHeight = ruleHeight + textPanelHeight + agentStatusHeight + queuedPanelHeight + modeAndHelpHeight + 1; // +1 for bottom padding
+        int nonScrollHeight = ruleHeight + agentStatusHeight + queuedPanelHeight + modeAndHelpHeight + 1; // +1 for bottom padding
         int scrollBottom = Math.Max(1, state.ConsoleHeight - nonScrollHeight);
 
         // If scroll region changed or a clear is needed, reset everything
@@ -455,52 +444,36 @@ public class HarnessAppComponent : ConsoleReactiveComponent<ConsoleReactiveProps
             System.Console.Write(AnsiEscapes.ResetScrollRegion);
             System.Console.Write(AnsiEscapes.EraseEntireScreen);
             System.Console.Write(AnsiEscapes.EraseScrollbackBuffer);
-            this._textScrollPanel.Reset();
-            this._resizedSinceLastRender = false;
 
             // Invalidate all children so they re-render even if props haven't changed
             this._rule.Invalidate();
             this._textScrollPanel.Invalidate();
-            this._textPanel.Invalidate();
             this._queuedPanel.Invalidate();
             this._agentStatus.Invalidate();
             this._modeAndHelp.Invalidate();
             this._textInput.Invalidate();
             this._listSelection.Invalidate();
+
+            this._resizedSinceLastRender = false;
         }
 
         this._scrollRegionBottom = scrollBottom;
 
         System.Console.Write(AnsiEscapes.SetScrollRegion(scrollBottom));
 
-        // Render text scroll panel in the scroll area (all items except the last)
-        IReadOnlyList<string> scrollItems = state.ScrollAreaContentItems.Count > 1
-            ? state.ScrollAreaContentItems.Take(state.ScrollAreaContentItems.Count - 1).ToList()
-            : [];
-
+        // Render text scroll panel in the scroll area
         this._textScrollPanel.Props = new TextScrollPanelProps
         {
             X = 1,
             Y = 1,
             Width = state.ConsoleWidth,
             Height = scrollBottom,
-            Items = scrollItems,
+            Items = state.ScrollAreaContentItems,
         };
         this._textScrollPanel.Render();
 
-        // Render the text panel for the last (dynamic) item just below the scroll region
-        this._textPanel.Props = new TextPanelProps
-        {
-            X = 1,
-            Y = scrollBottom + 1,
-            Width = state.ConsoleWidth,
-            Height = textPanelHeight,
-            Items = lastItems,
-        };
-        this._textPanel.Render();
-
-        // Render queued input items between text panel and agent status
-        int queuedPanelY = scrollBottom + textPanelHeight + 1;
+        // Render queued input items between scroll area and agent status
+        int queuedPanelY = scrollBottom + 1;
         this._queuedPanel.Props = new TextPanelProps
         {
             X = 1,