Phase 3: Fix diagnostics infrastructure tests

- Remove UpdateDisplay() wrapper, use Render.UpdateDisplay() directly
- Add BeginFrame() call to RenderCoordinator.UpdateDisplay()
- Fix test expectations for window borders (48x23 not 50x25)
- Enhance MockConsoleDriver with ConsoleBuffer for full rendering pipeline
- Fix diagnostics initialization order (before driver.Initialize())
- All 8 diagnostics infrastructure tests passing

Author: nickprotop

SharpConsoleUI.Tests/Infrastructure/MockConsoleDriver.cs

@@ -24,6 +24,7 @@ public class MockConsoleDriver : IConsoleDriver
 	private Point _cursorPosition;
 	private bool _cursorVisible;
 	private ConsoleWindowSystem? _windowSystem;
+	private ConsoleBuffer? _consoleBuffer;
 
 	/// <summary>
 	/// Gets the history of all WriteToConsole calls.
@@ -97,11 +98,11 @@ public void Clear()
 	}
 
 	/// <summary>
-	/// Does nothing (no buffering in mock driver).
+	/// Flushes the console buffer, triggering actual rendering with diagnostics capture.
 	/// </summary>
 	public void Flush()
 	{
-		// No-op for mock driver
+		_consoleBuffer?.Render();
 	}
 
 	/// <summary>
@@ -158,22 +159,34 @@ public void ResetCursorShape()
 	public void Initialize(ConsoleWindowSystem windowSystem)
 	{
 		_windowSystem = windowSystem;
+
+		// Create ConsoleBuffer for buffered rendering with diagnostics support
+		_consoleBuffer = new ConsoleBuffer(_screenSize.Width, _screenSize.Height, windowSystem.Options, null);
+
+		// Connect diagnostics to ConsoleBuffer
+		if (windowSystem.RenderingDiagnostics != null)
+		{
+			_consoleBuffer.Diagnostics = windowSystem.RenderingDiagnostics;
+		}
 	}
 
 	/// <summary>
-	/// Captures the output instead of writing to the console.
+	/// Writes content to the console buffer for buffered rendering.
 	/// </summary>
 	public void WriteToConsole(int x, int y, string value)
 	{
 		_outputHistory.Add(value);
+
+		// Add content to console buffer for double-buffered rendering
+		_consoleBuffer?.AddContent(x, y, value);
 	}
 
 	/// <summary>
-	/// Returns 0 (mock driver doesn't track dirty characters).
+	/// Returns the count of dirty characters from the ConsoleBuffer.
 	/// </summary>
 	public int GetDirtyCharacterCount()
 	{
-		return 0;
+		return _consoleBuffer?.GetDirtyCharacterCount() ?? 0;
 	}
 
 	/// <summary>

SharpConsoleUI.Tests/Rendering/Unit/DiagnosticsInfrastructureTests.cs

@@ -53,8 +53,9 @@ public void Diagnostics_CapturesCharacterBufferSnapshot()
 		// Assert
 		var snapshot = system.RenderingDiagnostics?.LastBufferSnapshot;
 		Assert.NotNull(snapshot);
-		Assert.Equal(50, snapshot.Width);
-		Assert.Equal(25, snapshot.Height);
+		// Window content buffer is 2 chars narrower/shorter due to borders
+		Assert.Equal(48, snapshot.Width);
+		Assert.Equal(23, snapshot.Height);
 	}
 
 	[Fact]
@@ -100,14 +101,15 @@ public void Diagnostics_MultipleFrames_RetainsHistory()
 		var window = new Window(system) { Width = 40, Height = 20 };
 		var markup = new MarkupControl(new List<string> { "Frame 1" });
 		window.AddControl(markup);
+		system.WindowStateService.AddWindow(window);
 
 		// Act - Render frame 1
-		window.RenderAndGetVisibleContent();
+		system.Render.UpdateDisplay();
 		var frame1Number = system.RenderingDiagnostics!.CurrentFrameNumber;
 
 		// Render frame 2 (invalidate and render again)
 		window.Invalidate(true);
-		window.RenderAndGetVisibleContent();
+		system.Render.UpdateDisplay();
 		var frame2Number = system.RenderingDiagnostics.CurrentFrameNumber;
 
 		// Assert
@@ -142,9 +144,10 @@ public void Diagnostics_CapturesMetrics()
 		var window = new Window(system) { Width = 40, Height = 20 };
 		var markup = new MarkupControl(new List<string> { "Test Metrics" });
 		window.AddControl(markup);
+		system.WindowStateService.AddWindow(window);
 
-		// Act
-		window.RenderAndGetVisibleContent();
+		// Act - Full render pipeline to trigger metrics capture
+		system.Render.UpdateDisplay();
 
 		// Assert
 		var metrics = system.RenderingDiagnostics?.LastMetrics;
@@ -160,9 +163,10 @@ public void Diagnostics_QualityAnalysis_GeneratesReport()
 		var window = new Window(system) { Width = 40, Height = 20 };
 		var markup = new MarkupControl(new List<string> { "Quality Test" });
 		window.AddControl(markup);
+		system.WindowStateService.AddWindow(window);
 
-		// Act
-		window.RenderAndGetVisibleContent();
+		// Act - Full render pipeline to trigger quality analysis
+		system.Render.UpdateDisplay();
 
 		// Assert
 		var qualityReport = system.RenderingDiagnostics?.LastQualityReport;

SharpConsoleUI/ConsoleWindowSystem.cs

@@ -225,7 +225,13 @@ public ConsoleWindowSystem(IConsoleDriver driver, ITheme theme, PluginConfigurat
 				Render,
 				() => this);
 
-			// NOW initialize driver with 'this' reference (after services exist)
+			// Initialize diagnostics BEFORE driver.Initialize() so driver can connect them
+			if (_options.EnableDiagnostics)
+			{
+				_renderingDiagnostics = new Diagnostics.RenderingDiagnostics(_options);
+			}
+
+			// NOW initialize driver with 'this' reference (after services and diagnostics exist)
 			_consoleDriver.Initialize(this);
 
 			// Set window system context on ThemeStateService for window invalidation
@@ -236,12 +242,6 @@ public ConsoleWindowSystem(IConsoleDriver driver, ITheme theme, PluginConfigurat
 			{
 				_pluginStateService.LoadPluginsFromDirectory(pluginConfiguration.GetEffectivePluginsDirectory());
 			}
-
-			// Initialize diagnostics if enabled (for testing and debugging)
-			if (_options.EnableDiagnostics)
-			{
-				_renderingDiagnostics = new Diagnostics.RenderingDiagnostics(_options);
-			}
 		}
 
 		#endregion
@@ -549,7 +549,7 @@ public int Run()
 						// Frame rate limiting enabled: only render if enough time elapsed
 						if (shouldRender && elapsed >= Performance.MinFrameTime)
 						{
-							UpdateDisplay();
+							Render.UpdateDisplay();
 							_lastRenderTime = now;
 							_idleTime = (int)Performance.MinFrameTime;
 						}
@@ -564,7 +564,7 @@ public int Run()
 						// Frame rate limiting disabled: render immediately when dirty
 						if (shouldRender)
 						{
-							UpdateDisplay();
+							Render.UpdateDisplay();
 							_lastRenderTime = now;
 							_idleTime = Configuration.SystemDefaults.FastLoopIdleMs; // Fast loop when dirty, no frame rate cap
 						}
@@ -643,7 +643,7 @@ public void Shutdown(int exitCode = 0)
 		public void ProcessOnce()
 		{
 			Input.ProcessInput();
-			UpdateDisplay();
+			Render.UpdateDisplay();
 			UpdateCursor();
 		}
 
@@ -792,11 +792,6 @@ private void UpdateCursor()
 			}
 		}
 
-		private void UpdateDisplay()
-		{
-			Render.UpdateDisplay();
-		}
-
 		private void UpdateStatusBarBounds()
 		{
 			Render.UpdateStatusBarBounds();

SharpConsoleUI/Rendering/RenderCoordinator.cs

@@ -186,6 +186,9 @@ public void AddWindowNeedingRegionUpdate(Window window)
 		/// </summary>
 		public void UpdateDisplay()
 		{
+			// Begin new frame for diagnostics tracking
+			_windowSystemContext.RenderingDiagnostics?.BeginFrame();
+
 			lock (_renderLock)
 			{
 				// ATOMIC DESKTOP CLEARING: Clear old window positions before rendering