Set window state for Raylib, make Frame occupy full size of parent UIElement or window.

Voile/Source/Rendering/RaylibRenderSystem.cs

@@ -81,7 +81,7 @@ namespace Voile.Rendering
                 Raylib.InitWindow((int)_windowSize.X, (int)_windowSize.Y, windowSettings.Title);
             }
 
-            // Raylib.SetWindowState(windowFlags);
+            Raylib.SetWindowState(windowFlags);
         }
 
         // TODO

Voile/Source/Rendering/RenderSystem.cs

@@ -256,7 +256,7 @@ namespace Voile.Rendering
     {
         public string Title;
         public Vector2 Size = new Vector2(1280, 720);
-        public bool Resizable { get; set; }
+        public bool Resizable { get; set; } = true;
 
         public WindowSettings(string title, Vector2 size)
         {

Voile/Source/UI/Containers/Container.cs

@@ -3,7 +3,6 @@ using Voile.Rendering;
 
 namespace Voile.UI.Containers;
 
-// TODO: make Container extend Widget, it already implements similar behaviors. 
 /// <summary>
 /// A base class for all UI containers, used to position and rendering child <see cref="IElement">s.
 /// </summary>

Voile/Source/UI/Containers/Frame.cs

@@ -1,5 +1,10 @@
+using Voile.Rendering;
+
 namespace Voile.UI.Containers;
 
+/// <summary>
+/// A frame is a special container that occupies the entire available size of the parent.
+/// </summary>
 public class Frame : Container
 {
     public Frame()
@@ -16,4 +21,37 @@ public class Frame : Container
     {
 
     }
+
+    public override void Render(RenderSystem renderer, Style style)
+    {
+        base.Render(renderer, style);
+
+        Rect parentSize;
+
+        if (Parent != null)
+        {
+            parentSize = Parent.Size;
+        }
+        else
+        {
+            var windowSize = renderer.WindowSize;
+            var windowRect = new Rect(windowSize.X, windowSize.Y);
+
+            parentSize = windowRect;
+        }
+
+        if (_lastParentSize != parentSize)
+        {
+            Size = parentSize;
+            _lastParentSize = parentSize;
+        }
+    }
+
+    protected override void OnUpdate()
+    {
+        base.OnUpdate();
+        Size = _lastParentSize;
+    }
+
+    private Rect _lastParentSize = Rect.Zero;
 }

Voile/Source/UI/UIElement.cs

@@ -10,6 +10,11 @@ public abstract class UIElement : IElement, IRenderableElement, IResizeableEleme
     public Vector2 LocalPosition { get; set; } = Vector2.Zero;
     public Vector2 GlobalPosition => _parent?.GlobalPosition + LocalPosition ?? LocalPosition;
 
+    /// <summary>
+    /// Parent <see cref="UIElement"/> of this element.
+    /// </summary>
+    public UIElement? Parent => _parent;
+
     public Rect Size
     {
         get => _size;