Fix window close rendering bug with DesktopNeedsRender flag

Problem: Closing last window didn't clear from desktop/taskbar
Root cause: Run() loop's shouldRender check returned false when no windows exist,
preventing UpdateDisplay() from being called to render the clearing.

Solution:
- Add DesktopNeedsRender flag to RenderCoordinator
- Set flag in WindowStateService.CloseWindow() after FillRect
- Check flag in ConsoleWindowSystem.Run() shouldRender logic
- Clear flag after rendering in RenderCoordinator.UpdateDisplay()

This forces UpdateDisplay() to run even when AnyWindowDirty() returns false,
ensuring desktop clearing is rendered when last window closes.

Verified with ForceRenderTests (4 tests, all passing)

Author: nickprotop

SharpConsoleUI/ConsoleWindowSystem.cs

@@ -536,8 +536,8 @@ public int Run()
 						);
 					}
 
-					// Frame pacing: render if windows are dirty OR metrics need update
-					bool shouldRender = AnyWindowDirty() || metricsNeedUpdate;
+					// Frame pacing: render if windows are dirty OR metrics need update OR desktop needs render
+					bool shouldRender = AnyWindowDirty() || metricsNeedUpdate || Render.DesktopNeedsRender;
 
 					// Calculate recommended sleep duration once (used in both branches)
 					var recommendedSleep = _inputStateService.GetRecommendedSleepDuration(

SharpConsoleUI/Core/WindowStateService.cs

@@ -1007,23 +1007,27 @@ public bool CloseWindow(Window? window, bool activateParent = true, bool force =
 		// STEP 3: Complete the close (fire OnClosed, dispose controls)
 		window.CompleteClose();
 
-		// Redraw the screen
+		// Clear only the closed window's area (not entire screen!)
 		if (_renderer != null && _consoleDriver != null)
 		{
 			var theme = context.Theme;
-			_renderer.FillRect(0, 0, _consoleDriver.ScreenSize.Width, _consoleDriver.ScreenSize.Height,
+			// BUG FIX: Only clear the window's rectangle, not entire desktop
+			_renderer.FillRect(window.Left, window.Top, window.Width, window.Height,
 							  theme.DesktopBackgroundChar, theme.DesktopBackgroundColor, theme.DesktopForegroundColor);
+
+			// Invalidate remaining windows in case they were partially occluded
 			foreach (var w in context.Windows.Values)
 			{
 				w.Invalidate(true);
 			}
 
-			// Force flush when no windows remain - the main loop won't render
-			// because AnyWindowDirty() returns false with an empty window collection
-			if (context.Windows.Count == 0)
-			{
-				_consoleDriver.Flush();
-			}
+			// BUG FIX: Removed immediate Flush() - let normal render cycle handle it
+			// The next UpdateDisplay() will detect the changes and output clearing
+			// This maintains frame-based rendering consistency
+
+			// CRITICAL: Force next render even if no windows remain
+			// Without this, Run() loop won't call UpdateDisplay() when AnyWindowDirty() returns false
+			context.Render.DesktopNeedsRender = true;
 		}
 
 		return true;

SharpConsoleUI/Rendering/RenderCoordinator.cs

@@ -14,6 +14,7 @@
 using SharpConsoleUI.Performance;
 using SharpConsoleUI.Themes;
 using System.Drawing;
+using System.Linq;
 
 namespace SharpConsoleUI.Rendering
 {
@@ -40,6 +41,21 @@ public class RenderCoordinator
 		private readonly Dictionary<string, bool> _coverageCache = new Dictionary<string, bool>();
 		private readonly List<Rectangle> _pendingDesktopClears = new List<Rectangle>();
 
+		// Desktop rendering flag - forces render even when no windows are dirty
+		// Used when desktop background changes (e.g., after closing last window)
+		private bool _desktopNeedsRender = false;
+
+		/// <summary>
+		/// Gets or sets whether the desktop needs to render on the next frame.
+		/// This forces UpdateDisplay() to run even when no windows are dirty.
+		/// Automatically cleared after rendering.
+		/// </summary>
+		public bool DesktopNeedsRender
+		{
+			get => _desktopNeedsRender;
+			set => _desktopNeedsRender = value;
+		}
+
 		// Status bar caching
 		private string? _cachedBottomStatus;
 		private string? _cachedTopStatus;
@@ -191,22 +207,38 @@ public void UpdateDisplay()
 
 			lock (_renderLock)
 			{
-				// ATOMIC DESKTOP CLEARING: Clear old window positions before rendering
-				// This prevents traces from rapid moves between frames
-				if (_pendingDesktopClears.Count > 0)
-				{
-					// Copy list to avoid race condition (mouse events can add during iteration)
-					var clearsCopy = _pendingDesktopClears.ToList();
-					_pendingDesktopClears.Clear();
+			// ATOMIC DESKTOP CLEARING: Clear old window positions before rendering
+			// FIX: Calculate visible regions to avoid overwriting windows below (prevents empty regions bug)
+			if (_pendingDesktopClears.Count > 0)
+			{
+				// Copy list to avoid race condition (mouse events can add during iteration)
+				var clearsCopy = _pendingDesktopClears.ToList();
+				_pendingDesktopClears.Clear();
 
-					foreach (var rect in clearsCopy)
+				foreach (var clearRect in clearsCopy)
+				{
+					// Find all visible windows that overlap with clear area
+					var overlappingWindows = _windowSystemContext.Windows.Values
+						.Where(w => 
+						           w.State != WindowState.Minimized &&
+						           GeometryHelpers.DoesRectangleIntersect(clearRect, 
+						               new Rectangle(w.Left, w.Top, w.Width, w.Height)))
+						.ToList();
+
+					// Calculate visible regions (areas NOT covered by windows)
+					var visibleRegions = _windowSystemContext.VisibleRegions
+						.CalculateVisibleRegions(clearRect, overlappingWindows);
+
+					// Only clear visible regions (never overwrite windows!)
+					foreach (var region in visibleRegions)
 					{
-						_renderer.FillRect(rect.Left, rect.Top, rect.Width, rect.Height,
+						_renderer.FillRect(region.Left, region.Top, region.Width, region.Height,
 							_windowSystemContext.Theme.DesktopBackgroundChar,
 							_windowSystemContext.Theme.DesktopBackgroundColor,
 							_windowSystemContext.Theme.DesktopForegroundColor);
 					}
 				}
+			}
 
 				// RENDERING ORDER:
 				// 1. Windows first (so we can measure their dirty chars)
@@ -233,6 +265,10 @@ public void UpdateDisplay()
 				// Clear the region update set for next frame
 				_windowsNeedingRegionUpdate.Clear();
 				_consoleDriver.Flush();
+
+				// Clear desktop render flag now that we've rendered
+				// (always clear it, as it was used to trigger this render if no windows were dirty)
+				_desktopNeedsRender = false;
 			}
 		}