Add project files.

Assets/Cryville/Common/Unity/UI/GridLayoutSizeFitter.cs

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+using UnityEngine;
+using UnityEngine.UI;
+
+namespace Cryville.Common.Unity.UI {
+	[RequireComponent(typeof(GridLayoutGroup))]
+	public class GridLayoutSizeFitter : MonoBehaviour {
+		RectTransform rectTransform;
+		GridLayoutGroup gridLayoutGroup;
+		Canvas canvas;
+		public int GroupItemCount = 3;
+
+#pragma warning disable IDE0051
+		void Awake() {
+			rectTransform = GetComponent<RectTransform>();
+			gridLayoutGroup = GetComponent<GridLayoutGroup>();
+			canvas = GetComponentInParent<Canvas>();
+		}
+		void Update() {
+			var cellSize = gridLayoutGroup.cellSize;
+			var frameSize = rectTransform.sizeDelta;
+			int lineCount = Mathf.CeilToInt((float)transform.childCount / GroupItemCount);
+			var rect = RectTransformUtility.PixelAdjustRect((RectTransform)transform, canvas);
+			if (gridLayoutGroup.startAxis == GridLayoutGroup.Axis.Horizontal) {
+				cellSize.x = rect.width / GroupItemCount;
+				frameSize.y = cellSize.y * lineCount;
+			}
+			else {
+				cellSize.y = rect.height / GroupItemCount;
+				frameSize.x = cellSize.x * lineCount;
+			}
+			gridLayoutGroup.cellSize = cellSize;
+			rectTransform.sizeDelta = frameSize;
+		}
+#pragma warning restore IDE0051
+	}
+}