fix(window): crash on rapid resize, atomic position+size update, bounds guards

- BorderRenderer: null guards and width snapshot to prevent cache race
  during rapid resize (fixes NullReferenceException in DrawVisibleBorders)
- Window: add SetPositionAndSize() for atomic position+size updates,
  prevents render thread from seeing partial state during drag/resize
- WindowPositioningManager: use SetPositionAndSize() in ResizeWindowTo
- ConsoleWindowSystem: fix hardcoded Max(1) to Max(0) in screen resize
  handler, guard DesktopBottomRight/DesktopDimensions against negative
  values when panels exceed screen height

Author: nickprotop

SharpConsoleUI/ConsoleWindowSystem.cs

@@ -460,12 +460,16 @@ public event EventHandler<Core.IdleStateEventArgs>? IdleStateChanged
 		/// <summary>
 		/// Gets the bottom-right coordinate of the usable desktop area (excluding status bars).
 		/// </summary>
-		public Point DesktopBottomRight => new Point(_consoleDriver.ScreenSize.Width - 1, _consoleDriver.ScreenSize.Height - 1 - Render.GetTopStatusHeight() - Render.GetBottomStatusHeight());
+		public Point DesktopBottomRight => new Point(
+			_consoleDriver.ScreenSize.Width - 1,
+			Math.Max(Render.GetTopStatusHeight(), _consoleDriver.ScreenSize.Height - 1 - Render.GetTopStatusHeight() - Render.GetBottomStatusHeight()));
 
 		/// <summary>
 		/// Gets the dimensions of the usable desktop area (excluding status bars).
 		/// </summary>
-		public Helpers.Size DesktopDimensions => new Helpers.Size(_consoleDriver.ScreenSize.Width, _consoleDriver.ScreenSize.Height - Render.GetTopStatusHeight() - Render.GetBottomStatusHeight());
+		public Helpers.Size DesktopDimensions => new Helpers.Size(
+			_consoleDriver.ScreenSize.Width,
+			Math.Max(0, _consoleDriver.ScreenSize.Height - Render.GetTopStatusHeight() - Render.GetBottomStatusHeight()));
 
 		/// <summary>
 		/// Gets the visible regions manager for calculating window visibility.
@@ -841,7 +845,7 @@ private void HandleScreenResize(object? sender, Size size)
 						}
 						if (window.Top + window.Height > desktopSize.Height)
 						{
-							window.Top = Math.Max(1, desktopSize.Height - window.Height);
+							window.Top = Math.Max(0, desktopSize.Height - window.Height);
 						}
 					}
 

SharpConsoleUI/Window.State.cs

@@ -553,6 +553,38 @@ internal void SetPositionDirect(Point point)
 			_top = point.Y;
 		}
 
+		/// <summary>
+		/// Sets position and size atomically, with a single invalidation pass.
+		/// Prevents the render thread from seeing partial state (old position with new size).
+		/// </summary>
+		internal void SetPositionAndSize(Point position, int width, int height)
+		{
+			// Apply constraints
+			if (_minimumWidth != null && width < _minimumWidth)
+				width = (int)_minimumWidth;
+			if (_minimumHeight != null && height < _minimumHeight)
+				height = (int)_minimumHeight;
+
+			bool changed = _left != position.X || _top != position.Y ||
+			               _width != width || _height != height;
+			if (!changed) return;
+
+			// Update all fields together
+			_left = position.X;
+			_top = position.Y;
+			_width = width;
+			_height = height;
+
+			// Single invalidation pass
+			InvalidateBorderCache();
+			Invalidate(true);
+
+			if (_scrollOffset > ContentLineCount - (Height - 2))
+			{
+				GoToBottom();
+			}
+		}
+
 		/// <summary>
 		/// Sets the window size with proper invalidation and constraint handling.
 		/// </summary>

SharpConsoleUI/Windows/BorderRenderer.cs

@@ -253,45 +253,52 @@ private void DrawVisibleBorders(List<Rectangle> visibleRegions)
 			var bg = _window.BackgroundColor;
 
 			// Rebuild cached border buffers if necessary
+			// Snapshot width to avoid race with resize changing _window.Width between check and use
+			var windowWidth = _window.Width;
 			if (_cachedTopBorder == null ||
-				_cachedBorderWidth != _window.Width ||
+				_cachedBorderWidth != windowWidth ||
 				_cachedBorderIsActive != isActive)
 			{
 				_cachedTopBorder = BuildTopBorder(chars, borderFg, titleFg, buttonFg, closeFg, bg);
 				_cachedBottomBorder = BuildBottomBorder(chars, borderFg, bg);
-				_cachedBorderWidth = _window.Width;
+				_cachedBorderWidth = windowWidth;
 				_cachedBorderIsActive = isActive;
 			}
 
 			var contentHeight = _window.TotalLines;
 			var visibleHeight = _window.Height - 2;
 			var scrollbarVisible = _window.IsScrollable && contentHeight > visibleHeight;
+			var cachedTop = _cachedTopBorder;
+			var cachedBottom = _cachedBottomBorder;
 
 			foreach (var region in visibleRegions ?? new List<Rectangle>())
 			{
 				// Top border
-				if (region.Top == _window.Top)
+				if (region.Top == _window.Top && cachedTop != null)
 				{
 					int borderStartX = Math.Max(region.Left, _window.Left);
-					int borderWidth = Math.Min(region.Width, _window.Left + _window.Width - borderStartX);
+					int borderWidth = Math.Min(region.Width, _window.Left + windowWidth - borderStartX);
+					// Clamp to cached buffer width in case of resize race
+					borderWidth = Math.Min(borderWidth, cachedTop.Width - Math.Max(0, borderStartX - _window.Left));
 					if (borderWidth > 0)
 					{
 						int srcX = borderStartX - _window.Left;
 						driver.WriteBufferRegion(borderStartX, region.Top + desktopUpperLeft.Y,
-							_cachedTopBorder, srcX, 0, borderWidth, bg);
+							cachedTop, srcX, 0, borderWidth, bg);
 					}
 				}
 
 				// Bottom border
-				if (region.Top + region.Height == _window.Top + _window.Height)
+				if (region.Top + region.Height == _window.Top + _window.Height && cachedBottom != null)
 				{
 					int borderStartX = Math.Max(region.Left, _window.Left);
-					int borderWidth = Math.Min(region.Width, _window.Left + _window.Width - borderStartX);
+					int borderWidth = Math.Min(region.Width, _window.Left + windowWidth - borderStartX);
+					borderWidth = Math.Min(borderWidth, cachedBottom.Width - Math.Max(0, borderStartX - _window.Left));
 					if (borderWidth > 0)
 					{
 						int srcX = borderStartX - _window.Left;
 						driver.WriteBufferRegion(borderStartX, _window.Top + _window.Height - 1 + desktopUpperLeft.Y,
-							_cachedBottomBorder!, srcX, 0, borderWidth, bg);
+							cachedBottom, srcX, 0, borderWidth, bg);
 					}
 				}
 			}

SharpConsoleUI/Windows/WindowPositioningManager.cs

@@ -123,10 +123,9 @@ public void ResizeWindowTo(Window window, int newLeft, int newTop, int newWidth,
 				// Add old bounds to pending clears
 				_renderCoordinator.AddPendingDesktopClear(oldBounds);
 
-				// Apply the new position and size
-				window.SetPositionDirect(new Point(newLeft, newTop));
-				window.SetSize(newWidth, newHeight);
-				window.Invalidate(true);
+				// Apply position and size atomically (prevents render thread from
+				// seeing partial state between position and size changes)
+				window.SetPositionAndSize(new Point(newLeft, newTop), newWidth, newHeight);
 
 				// Invalidate windows that were underneath (now exposed)
 				InvalidateExposedRegions(window, oldBounds);