feat(drawings): explain an empty list that Hide Fully Nested Drawings emptied

With the filter on and every drawing nested, the Drawings tab was a blank
list with no hint why. It now shows a note that turns the filter off when
clicked. The filter is never turned off on its own: it is the user's
choice and would stay off when new drawings arrive.

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

OpenNest.WinForms.Tests/Controls/HideNestedDrawingsNoteTests.cs

@@ -0,0 +1,90 @@
+using System.Drawing;
+using System.Reflection;
+using System.Windows.Forms;
+using OpenNest.CNC;
+using OpenNest.Controls;
+using OpenNest.Forms;
+
+namespace OpenNest.WinForms.Tests.Controls;
+
+[Collection("Fill operation lifetime")]
+public class HideNestedDrawingsNoteTests
+{
+    [Fact]
+    public void HidingEveryDrawingShowsANoteThatTurnsTheFilterOff() => RunSta(() =>
+    {
+        var nest = new Nest("hide");
+        var drawing = new Drawing("a", Square());
+        drawing.Quantity.Required = 1;
+        nest.Drawings.Add(drawing);
+        var plate = new Plate(100, 100) { Quantity = 1 };
+        nest.Plates.Add(plate);
+        plate.Parts.Add(new Part(drawing));
+        Assert.Equal(0, drawing.Quantity.Remaining);
+
+        using var form = new EditNestForm(nest) { Width = 1100, Height = 700 };
+        form.PlateView.SetOverlapAutoCheck(null);
+        form.Show();
+        Application.DoEvents();
+
+        var list = Field<DrawingListBox>(form, "drawingListBox1");
+        var hide = Field<ToolStripButton>(form, "hideNestedButton");
+        Assert.Null(list.EmptyText);
+
+        hide.Checked = true;
+
+        Assert.Empty(list.Items);
+        Assert.NotNull(list.EmptyText);
+
+        typeof(DrawingListBox)
+            .GetMethod("OnMouseUp", BindingFlags.Instance | BindingFlags.NonPublic)!
+            .Invoke(list, [new MouseEventArgs(MouseButtons.Left, 1, 20, 20, 0)]);
+
+        Assert.False(hide.Checked);
+        Assert.Single(list.Items);
+        Assert.Null(list.EmptyText);
+    });
+
+    [Fact]
+    public void ListWithRowsNeverRaisesTheNoteClick() => RunSta(() =>
+    {
+        using var list = new ClickableDrawingListBox { Size = new Size(260, 400), EmptyText = "note" };
+        list.Items.Add(new Drawing("a", Square()));
+        var clicks = 0;
+        list.EmptyTextClicked += (_, _) => clicks++;
+
+        list.Release(new MouseEventArgs(MouseButtons.Left, 1, 20, 300, 0));
+        Assert.Equal(0, clicks);
+
+        list.Items.Clear();
+        list.Release(new MouseEventArgs(MouseButtons.Right, 1, 20, 20, 0));
+        Assert.Equal(0, clicks);
+
+        list.Release(new MouseEventArgs(MouseButtons.Left, 1, 20, 20, 0));
+        Assert.Equal(1, clicks);
+    });
+
+    private static T Field<T>(EditNestForm form, string name) =>
+        (T)typeof(EditNestForm)
+            .GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!
+            .GetValue(form)!;
+
+    private static Program Square()
+    {
+        var program = new Program();
+        program.MoveTo(0, 0);
+        program.LineTo(0, 10);
+        program.LineTo(10, 10);
+        program.LineTo(10, 0);
+        program.LineTo(0, 0);
+        return program;
+    }
+
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromMinutes(3), "The STA test did not complete.");
+
+    private sealed class ClickableDrawingListBox : DrawingListBox
+    {
+        public void Release(MouseEventArgs e) => OnMouseUp(e);
+    }
+}

OpenNest/Controls/DrawingListBox.cs

@@ -8,6 +8,7 @@ namespace OpenNest.Controls
 {
     public class DrawingListBox : ListBox
     {
+        private const int WM_PAINT = 0x000F;
         private const int WM_ERASEBKGND = 0x0014;
 
         private readonly Size imageSize;
@@ -115,6 +116,30 @@ namespace OpenNest.Controls
         /// </summary>
         public Plate Plate { get; set; }
 
+        private string emptyText;
+
+        /// <summary>
+        /// A note drawn in the middle of the list while it has no rows; null draws nothing.
+        /// Clicking the empty list while the note shows raises <see cref="EmptyTextClicked"/>.
+        /// </summary>
+        public string EmptyText
+        {
+            get => emptyText;
+            set
+            {
+                if (emptyText == value)
+                    return;
+
+                emptyText = value;
+                Invalidate();
+            }
+        }
+
+        /// <summary>Raised when the empty list is clicked while <see cref="EmptyText"/> shows.</summary>
+        public event EventHandler EmptyTextClicked;
+
+        private bool IsShowingEmptyText => Items.Count == 0 && !string.IsNullOrEmpty(emptyText);
+
         protected override void OnDrawItem(DrawItemEventArgs e)
         {
             if (e.Index >= Items.Count || e.Index <= -1)
@@ -227,6 +252,8 @@ namespace OpenNest.Controls
         {
             base.OnMouseMove(e);
 
+            Cursor = IsShowingEmptyText && EmptyTextClicked != null ? Cursors.Hand : Cursors.Default;
+
             var drawing = pressedDrawing;
 
             if (drawing == null || e.Button != MouseButtons.Left)
@@ -263,6 +290,36 @@ namespace OpenNest.Controls
         {
             pressedDrawing = null;
             base.OnMouseUp(e);
+
+            if (e.Button == MouseButtons.Left && IsShowingEmptyText)
+                EmptyTextClicked?.Invoke(this, EventArgs.Empty);
+        }
+
+        protected override void OnResize(EventArgs e)
+        {
+            base.OnResize(e);
+
+            // The note wraps to the width, so a resize must repaint all of it.
+            if (IsShowingEmptyText)
+                Invalidate();
+        }
+
+        private void DrawEmptyText()
+        {
+            using var g = CreateGraphics();
+            var bounds = Rectangle.Inflate(ClientRectangle, -12, -12);
+
+            TextRenderer.DrawText(
+                g,
+                emptyText,
+                Font,
+                bounds,
+                SystemColors.GrayText,
+                TextFormatFlags.HorizontalCenter
+                    | TextFormatFlags.Top
+                    | TextFormatFlags.WordBreak
+                    | TextFormatFlags.TextBoxControl
+            );
         }
 
         /// <summary>
@@ -310,6 +367,10 @@ namespace OpenNest.Controls
                 return;
             }
             base.WndProc(ref m);
+
+            // An owner-drawn list paints nothing when empty, so the note goes on top afterward.
+            if (m.Msg == WM_PAINT && IsShowingEmptyText)
+                DrawEmptyText();
         }
     }
 

OpenNest/Forms/EditNestForm.cs

@@ -479,6 +479,7 @@ namespace OpenNest.Forms
             drawingListBox1.OpenInConverterRequested += drawingListBox1_OpenInConverterRequested;
             drawingListBox1.DrawingPickedUp += drawingListBox1_DrawingPickedUp;
             drawingListBox1.DrawingDropped += drawingListBox1_DrawingDropped;
+            drawingListBox1.EmptyTextClicked += (_, _) => hideNestedButton.Checked = false;
             Activated += (_, _) => UpdateDrawingLibraryCommand();
             UpdateDrawingLibraryCommand();
             PlateView.BeginChangeHandler = BeginChange;
@@ -584,6 +585,20 @@ namespace OpenNest.Forms
                 drawingListBox1.TopIndex = topIndex;
 
             drawingListBox1.EndUpdate();
+            UpdateDrawingListEmptyText();
+        }
+
+        /// <summary>
+        /// Explains an empty list that only Hide Fully Nested Drawings emptied, so the drawings
+        /// don't look lost; clicking the note turns the filter off. The filter stays the user's
+        /// choice and is never turned off on its own.
+        /// </summary>
+        private void UpdateDrawingListEmptyText()
+        {
+            drawingListBox1.EmptyText =
+                hideNestedButton.Checked && Nest.Drawings.Count > 0
+                    ? "All drawings are fully nested and hidden.\n\nClick here to show them."
+                    : null;
         }
 
         public void Save()
@@ -1827,6 +1842,7 @@ namespace OpenNest.Forms
                         drawingListBox1.EndUpdate();
                     }
 
+                    UpdateDrawingListEmptyText();
                     drawingListBox1.Invalidate();
 
                     // Parts were added or removed: rebuild the plate rows so the