move class into its own file

some questionable nullable annotations whatever

Author: Morilli

src/BizHawk.Client.EmuHawk/RetroAchievements/RCheevosAchievementListForm.cs

@@ -3,8 +3,6 @@
 using System.Linq;
 using System.Windows.Forms;
 
-using BizHawk.Common.NumberExtensions;
-
 namespace BizHawk.Client.EmuHawk
 {
 	/// <summary>
@@ -115,31 +113,4 @@ private void RCheevosAchievementListForm_SizeChanged(object sender, EventArgs e)
 			UpdateForms();
 		}
 	}
-
-	public class VirtualizedFlowLayoutPanel : FlowLayoutPanel
-	{
-		private VScrollBar _boundScrollBar;
-		public VScrollBar BoundScrollBar
-		{
-			get => _boundScrollBar;
-			set
-			{
-				_boundScrollBar = value;
-				_boundScrollBar.SmallChange = 5;
-				_boundScrollBar.LargeChange = this.Height;
-			}
-		}
-
-		protected override void OnSizeChanged(EventArgs e)
-		{
-			if (_boundScrollBar is not null)
-				_boundScrollBar.LargeChange = this.Height;
-			base.OnSizeChanged(e);
-		}
-
-		protected override void OnMouseWheel(MouseEventArgs e)
-		{
-			BoundScrollBar.Value = (BoundScrollBar.Value - e.Delta).Clamp(BoundScrollBar.Minimum, BoundScrollBar.Maximum - BoundScrollBar.LargeChange + 1);
-		}
-	}
 }

src/BizHawk.WinForms.Controls/FLPs/VirtualizedFlowLayoutPanel.cs

@@ -0,0 +1,32 @@
+using System.Windows.Forms;
+
+using BizHawk.Common.NumberExtensions;
+
+namespace BizHawk.Client.EmuHawk;
+
+public class VirtualizedFlowLayoutPanel : FlowLayoutPanel
+{
+	private VScrollBar? _boundScrollBar;
+	public VScrollBar BoundScrollBar
+	{
+		get => _boundScrollBar!;
+		set
+		{
+			_boundScrollBar = value;
+			_boundScrollBar.SmallChange = 5;
+			_boundScrollBar.LargeChange = this.Height;
+		}
+	}
+
+	protected override void OnSizeChanged(EventArgs e)
+	{
+		if (_boundScrollBar is not null)
+			_boundScrollBar.LargeChange = this.Height;
+		base.OnSizeChanged(e);
+	}
+
+	protected override void OnMouseWheel(MouseEventArgs e)
+	{
+		BoundScrollBar.Value = (BoundScrollBar.Value - e.Delta).Clamp(BoundScrollBar.Minimum, BoundScrollBar.Maximum - BoundScrollBar.LargeChange + 1);
+	}
+}