Fix any remaining bugs with anchor positioning system, use LocalPosition for UIElement, and make containers use that for arrangement.

Voile/Source/UI/UIElement.cs

@@ -7,7 +7,9 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
 {
     public bool Visible { get; set; } = true;
     public bool IgnoreInput { get; set; } = false;
-    public Vector2 Position { get; set; } = Vector2.Zero;
+    public Vector2 LocalPosition { get; set; } = Vector2.Zero;
+    public Vector2 GlobalPosition => _parent?.GlobalPosition + LocalPosition ?? LocalPosition;
+
     public Rect Size
     {
         get => _size;
@@ -36,6 +38,11 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
 
     public virtual void MarkDirty() => _dirty = true;
 
+    public void SetParent(UIElement parent)
+    {
+        _parent = parent;
+    }
+
     public void Update()
     {
         if (!_dirty) return;
@@ -46,9 +53,9 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
 
         OnUpdate();
 
-        if (_parentRect != Rect.Zero)
+        if (_parent is not null && _parent.Size != Rect.Zero)
         {
-            ApplyAnchor(_parentPosition, _parentRect);
+            ApplyAnchor(_parent.GlobalPosition, _parent.Size);
         }
     }
 
@@ -57,7 +64,7 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
 
     public void DrawSize(RenderSystem renderer)
     {
-        renderer.SetTransform(Position, Vector2.Zero);
+        renderer.SetTransform(GlobalPosition, Vector2.Zero);
         renderer.DrawRectangleOutline(new Vector2(Size.Width, Size.Height), Color.Red, 2.0f);
     }
 
@@ -68,9 +75,9 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
     /// <returns>True if the point is inside the widget; otherwise, false.</returns>
     public bool ContainsPoint(Vector2 point)
     {
-        return point.X >= Position.X && point.Y >= Position.Y &&
-               point.X <= Position.X + Size.Width &&
-               point.Y <= Position.Y + Size.Height;
+        return point.X >= GlobalPosition.X && point.Y >= GlobalPosition.Y &&
+               point.X <= GlobalPosition.X + Size.Width &&
+               point.Y <= GlobalPosition.Y + Size.Height;
     }
 
     /// <summary>
@@ -78,14 +85,11 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
     /// </summary>
     public virtual void ApplyAnchor(Vector2 parentPosition, Rect parentRect)
     {
-        _parentPosition = parentPosition;
-        _parentRect = parentRect;
-        Position = Anchor.Calculate(parentPosition, parentRect, Size) + new Vector2(AnchorOffset.X, AnchorOffset.Y);
+        LocalPosition = Anchor.Calculate(parentPosition, parentRect, Size) + new Vector2(AnchorOffset.X, AnchorOffset.Y);
     }
 
     private bool _dirty = true;
     private Rect _size = Rect.Zero;
 
-    private Vector2 _parentPosition = Vector2.Zero;
-    private Rect _parentRect = Rect.Zero;
+    private UIElement? _parent;
 }