Add restore glyph and support for restored window state in title bar

src/Modern.Forms/ControlPaint.cs

@@ -106,6 +106,41 @@ public static void DrawMaximizeGlyph (PaintEventArgs e, Rectangle rectangle)
             e.Canvas.DrawRectangle (rectangle, Theme.ForegroundColorOnAccent);
         }
 
+        /// <summary>
+        /// Draws a restore glyph, as seen on FormTitleBar when the window is maximized.
+        /// </summary>
+        public static void DrawRestoreGlyph (PaintEventArgs e, Rectangle rectangle)
+        {
+            var color = Theme.ForegroundColorOnAccent;
+            var offset = e.LogicalToDeviceUnits (2);
+
+            var back = new Rectangle (
+                rectangle.X + offset,
+                rectangle.Y,
+                rectangle.Width - offset - 1,
+                rectangle.Height - offset - 1);
+
+            var front = new Rectangle (
+                rectangle.X,
+                rectangle.Y + offset,
+                rectangle.Width - offset,
+                rectangle.Height - offset);
+
+            // Draw "front" "window"
+            e.Canvas.DrawRectangle (front, color);
+
+            // Draw "back" "window"
+            using var path = new SKPath ();
+
+            path.MoveTo (back.Left, front.Top);
+            path.LineTo (back.Left, back.Top);
+            path.LineTo (back.Right, back.Top);
+            path.LineTo (back.Right, back.Bottom);
+            path.LineTo (front.Right, back.Bottom);
+
+            e.Canvas.DrawPath (path, color);
+        }
+
         /// <summary>
         /// Draws a minimize glyph, as seen on FormTitleBar.
         /// </summary>
@@ -114,7 +149,6 @@ public static void DrawMinimizeGlyph (PaintEventArgs e, Rectangle rectangle)
             e.Canvas.DrawLine (rectangle.X, rectangle.Y, rectangle.Right, rectangle.Y, Theme.ForegroundColorOnAccent);
         }
 
-
         /// <summary>
         /// Draws a RadioButton glyph.
         /// </summary>

src/Modern.Forms/Extensions/SkiaExtensions.cs

@@ -144,6 +144,16 @@ public static void DrawLine (this SKCanvas canvas, float x1, float y1, float x2,
             canvas.DrawLine (x1, y1, x2, y2, paint);
         }
 
+        /// <summary>
+        /// Draws a path.
+        /// </summary>
+        public static void DrawPath (this SKCanvas canvas, SKPath path, SKColor color, int thickness = 1)
+        {
+            using var paint = new SKPaint { Color = color, StrokeWidth = thickness, IsStroke = true };
+
+            canvas.DrawPath (path, paint);
+        }
+
         /// <summary>
         /// Draws an unfilled rectangle.
         /// </summary>

src/Modern.Forms/FormTitleBar.cs

@@ -30,16 +30,21 @@ public FormTitleBar ()
             minimize_button.Click += (o, e) => {
                 var form_min = FindForm ();
 
-                if (form_min != null)
+                if (form_min is not null)
                     form_min.WindowState = FormWindowState.Minimized;
             };
 
             maximize_button = Controls.AddImplicitControl (new TitleBarButton (TitleBarButton.TitleBarButtonGlyph.Maximize));
             maximize_button.Click += (o, e) => {
                 var form = FindForm ();
 
-                if (form != null)
-                    form.WindowState = form.WindowState == FormWindowState.Maximized ? FormWindowState.Normal : FormWindowState.Maximized;
+                if (form is not null) {
+                    form.WindowState = form.WindowState == FormWindowState.Maximized
+                        ? FormWindowState.Normal
+                        : FormWindowState.Maximized;
+
+                    UpdateMaximizeButtonGlyph ();
+                }
             };
 
             close_button = Controls.AddImplicitControl (new TitleBarButton (TitleBarButton.TitleBarButtonGlyph.Close));
@@ -64,6 +69,7 @@ public bool AllowMaximize {
             get => maximize_button.Visible;
             set {
                 maximize_button.Visible = value;
+                UpdateMaximizeButtonGlyph ();
                 Invalidate (); // TODO: Shouldn't be necessary, should automatically be triggered
             }
         }
@@ -126,6 +132,8 @@ protected override void OnSizeChanged (EventArgs e)
 
             // Keep our form image a square
             form_image.Width = Height;
+
+            UpdateMaximizeButtonGlyph ();
         }
 
         /// <summary>
@@ -136,7 +144,7 @@ public bool ShowImage {
             set {
                 if (show_image != value) {
                     show_image = value;
-                    form_image.Visible = value && form_image is not null;
+                    form_image.Visible = value && form_image.Image is not null;
                     Invalidate (); // TODO: Shouldn't be required
                 }
             }
@@ -145,11 +153,38 @@ public bool ShowImage {
         /// <inheritdoc/>
         public override ControlStyle Style { get; } = new ControlStyle (DefaultStyle);
 
+        private void UpdateMaximizeButtonGlyph ()
+        {
+            var form = FindForm ();
+
+            if (form == null || !maximize_button.Visible) {
+                maximize_button.Glyph = TitleBarButton.TitleBarButtonGlyph.Maximize;
+                return;
+            }
+
+            maximize_button.Glyph = form.WindowState == FormWindowState.Maximized
+                ? TitleBarButton.TitleBarButtonGlyph.Restore
+                : TitleBarButton.TitleBarButtonGlyph.Maximize;
+        }
+
         internal sealed class TitleBarButton : Button
         {
             private const int BUTTON_PADDING = 10;
 
-            private readonly TitleBarButtonGlyph glyph;
+            private TitleBarButtonGlyph glyph;
+
+            /// <summary>
+            /// Gets or sets the glyph displayed by the button.
+            /// </summary>
+            public TitleBarButtonGlyph Glyph {
+                get => glyph;
+                set {
+                    if (glyph != value) {
+                        glyph = value;
+                        Invalidate ();
+                    }
+                }
+            }
 
             public TitleBarButton (TitleBarButtonGlyph glyph)
             {
@@ -183,14 +218,21 @@ protected override void OnPaint (PaintEventArgs e)
                     case TitleBarButtonGlyph.Maximize:
                         ControlPaint.DrawMaximizeGlyph (e, glyph_bounds);
                         break;
+                    case TitleBarButtonGlyph.Restore:
+                        ControlPaint.DrawRestoreGlyph (e, glyph_bounds);
+                        break;
                 }
             }
 
+            /// <summary>
+            /// Specifies which glyph is displayed by the title bar button.
+            /// </summary>
             public enum TitleBarButtonGlyph
             {
                 Close,
                 Minimize,
-                Maximize
+                Maximize,
+                Restore
             }
         }
     }