Resolve a TODO in `InputRoll.RowCount`

src/BizHawk.Client.EmuHawk/CustomControls/InputRoll/Cell.cs

@@ -1,5 +1,6 @@
 #nullable enable
 
+using System.Collections.Generic;
 using System.Diagnostics;
 
 using BizHawk.Common;
@@ -91,6 +92,11 @@ namespace BizHawk.Client.EmuHawk
 
 	public sealed class CellList : SortedList<Cell>
 	{
+		public CellList() {}
+
+		public CellList(IEnumerable<Cell> collection)
+			: base(collection) {}
+
 		/// <remarks>restore the distinctness invariant from <see cref="System.Collections.Generic.SortedSet{T}"/>; though I don't think we actually rely on it anywhere --yoshi</remarks>
 		public override void Add(Cell item)
 		{
@@ -117,6 +123,9 @@ namespace BizHawk.Client.EmuHawk
 			return i >= 0 && _list[i].RowIndex == rowIndex;
 		}
 #endif
+
+		public new CellList Slice(int start, int length)
+			=> new(SliceImpl(start: start, length: length));
 	}
 
 	public static class CellExtensions

src/BizHawk.Client.EmuHawk/CustomControls/InputRoll/InputRoll.cs

@@ -22,7 +22,7 @@ namespace BizHawk.Client.EmuHawk
 	{
 		private readonly IControlRenderer _renderer;
 
-		private readonly CellList _selectedItems = new();
+		private CellList _selectedItems = new();
 
 		// scrollbar location(s) are calculated later (e.g. on resize)
 		private readonly VScrollBar _vBar = new VScrollBar { Visible = false };
@@ -269,10 +269,10 @@ namespace BizHawk.Client.EmuHawk
 
 					_rowCount = value;
 
-					//TODO replace this with a binary search + truncate
 					if (_selectedItems.LastOrDefault()?.RowIndex >= _rowCount)
 					{
-						_selectedItems.RemoveAll(i => i.RowIndex >= _rowCount);
+						var iLastToKeep = _selectedItems.LowerBoundBinarySearch(static c => c.RowIndex ?? -1, _rowCount);
+						_selectedItems = _selectedItems.Slice(start: 0, length: iLastToKeep + 1);
 					}
 
 					RecalculateScrollBars();

src/BizHawk.Common/CustomCollections.cs

@@ -1,5 +1,6 @@
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace BizHawk.Common
 {
@@ -14,11 +15,15 @@ namespace BizHawk.Common
 
 		public virtual bool IsReadOnly { get; } = false;
 
-		public SortedList() => _list = new List<T>();
+		protected SortedList(List<T> wrapped)
+			=> _list = wrapped;
+
+		public SortedList()
+			: this(new()) {}
 
 		public SortedList(IEnumerable<T> collection)
+			: this(new(collection))
 		{
-			_list = new List<T>(collection);
 			_list.Sort();
 		}
 
@@ -131,6 +136,12 @@ namespace BizHawk.Common
 			}
 		}
 
+		public SortedList<T> Slice(int start, int length)
+			=> new(SliceImpl(start: start, length: length));
+
+		protected List<T> SliceImpl(int start, int length)
+			=> _list.Skip(start).Take(length).ToList();
+
 		IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 	}
 }