fix(drawing-list): paint the whole blank area when the side panel resizes

Windows erases a resized window before WinForms updates its cached
ClientSize, so the erase handler filled the blank area below the rows only
out to the old width. Windows then treated the new strip as erased, and it
kept whatever was on screen before. The handler now reads the client rect
from Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest/Controls/DrawingListBox.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Drawing;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Windows.Forms;
 using Size = System.Drawing.Size;
 
@@ -340,26 +341,32 @@ namespace OpenNest.Controls
         {
             if (m.Msg == WM_ERASEBKGND)
             {
+                // Windows erases a resized window before WinForms hears the new size, so the
+                // cached ClientSize still holds the old one; filling to it left the newly
+                // exposed strip unpainted (and validated, so nothing repainted it later).
+                GetClientRect(Handle, out var client);
+                var clientWidth = client.Right - client.Left;
+                var clientHeight = client.Bottom - client.Top;
                 var itemBottom = 0;
 
                 if (Items.Count > 0)
                 {
                     var lastVisible = System.Math.Min(
-                        TopIndex + (ClientSize.Height / ItemHeight),
+                        TopIndex + (clientHeight / ItemHeight),
                         Items.Count - 1
                     );
                     itemBottom = GetItemRectangle(lastVisible).Bottom;
                 }
 
-                if (itemBottom < ClientSize.Height)
+                if (itemBottom < clientHeight)
                 {
                     using var g = Graphics.FromHdc(m.WParam);
                     g.FillRectangle(
                         Brushes.White,
                         0,
                         itemBottom,
-                        ClientSize.Width,
-                        ClientSize.Height - itemBottom
+                        clientWidth,
+                        clientHeight - itemBottom
                     );
                 }
 
@@ -372,6 +379,19 @@ namespace OpenNest.Controls
             if (m.Msg == WM_PAINT && IsShowingEmptyText)
                 DrawEmptyText();
         }
+
+        [StructLayout(LayoutKind.Sequential)]
+        private struct RECT
+        {
+            public int Left;
+            public int Top;
+            public int Right;
+            public int Bottom;
+        }
+
+        [DllImport("user32.dll")]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        private static extern bool GetClientRect(IntPtr hWnd, out RECT rect);
     }
 
     public static class PointExtensions