Add MarginContainer, mark parent elements dirty when child gets marked dirty too.

Voile/Source/UI/Containers/Container.cs

@@ -60,6 +60,18 @@ public abstract class Container : UIElement, IParentableElement
         }
     }
 
+    public override void MarkDirty()
+    {
+        base.MarkDirty();
+
+        foreach (var child in _children)
+        {
+            if (child is not IUpdatableElement updatable) continue;
+
+            updatable.MarkDirty();
+        }
+    }
+
     /// <summary>
     /// Called when this <see cref="Container"/> has to rearrange its children.
     /// </summary>

Voile/Source/UI/Containers/FillContainer.cs

@@ -51,9 +51,6 @@ public class FillContainer : Container
     protected override void OnUpdate()
     {
         base.OnUpdate();
-
-        if (Size == _lastParentSize) return;
-
         Size = _lastParentSize;
     }
 

Voile/Source/UI/Containers/MarginContainer.cs

@@ -0,0 +1,78 @@
+using System.Numerics;
+
+namespace Voile.UI.Containers;
+
+/// <summary>
+/// Represents the margin offsets applied around an element.
+/// </summary>
+public struct Margin
+{
+    public float Left;
+    public float Right;
+    public float Top;
+    public float Bottom;
+
+    public Margin(float uniform)
+    {
+        Left = Right = Top = Bottom = uniform;
+    }
+
+    public Margin(float horizontal, float vertical)
+    {
+        Left = Right = horizontal;
+        Top = Bottom = vertical;
+    }
+
+    public Margin(float left, float right, float top, float bottom)
+    {
+        Left = left;
+        Right = right;
+        Top = top;
+        Bottom = bottom;
+    }
+
+    public static Margin Zero => new Margin(0);
+}
+
+public class MarginContainer : Container
+{
+    /// <summary>
+    /// The margin to apply around the contents of this container.
+    /// </summary>
+    public Margin Margin { get; set; }
+
+    /// <summary>
+    /// Specifies if this <see cref="MarginContainer"/> will fill to parent size.
+    /// </summary>
+    public bool Fill { get; set; } = true;
+
+    public MarginContainer() : this(new Margin()) { }
+
+    public MarginContainer(Margin margin)
+    {
+        Margin = margin;
+    }
+
+    protected override void OnUpdate()
+    {
+        base.OnUpdate();
+        if (Parent == null) return;
+
+        Size = Parent.Size;
+    }
+
+    public override void Arrange()
+    {
+        foreach (var child in Children)
+        {
+            var newPosition = new Vector2(Margin.Left, Margin.Top);
+            var newSize = new Rect(
+                Size.Width - Margin.Left - Margin.Right,
+                Size.Height - Margin.Top - Margin.Bottom
+            );
+
+            child.Size = newSize;
+            child.LocalPosition = newPosition;
+        }
+    }
+}

Voile/Source/UI/UIElement.cs

@@ -23,8 +23,6 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
         get => _size;
         set
         {
-            _size = value;
-
             if (value.Width < MinimumSize.Width)
             {
                 _size.Width = MinimumSize.Width;
@@ -35,7 +33,12 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
                 _size.Height = MinimumSize.Height;
             }
 
-            MarkDirty();
+            if (_size != value)
+            {
+                MarkDirty();
+            }
+
+            _size = value;
         }
     }
     public Vector2 AnchorOffset { get; set; } = Vector2.Zero;
@@ -44,7 +47,15 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
     public abstract Rect MinimumSize { get; }
     public bool Dirty => _dirty;
 
-    public virtual void MarkDirty() => _dirty = true;
+    public virtual void MarkDirty()
+    {
+        if (Parent != null && !Parent.Dirty)
+        {
+            Parent.MarkDirty();
+        }
+
+        _dirty = true;
+    }
 
     /// <summary>
     /// Sets a parent element for this <see cref="UIElement"/>.