Fix VirtualisedListContainer rows not returning child to pool on disposal

osu.Framework.Tests/Visual/Containers/TestSceneVirtualisedListContainer.cs

@@ -37,15 +37,20 @@ public void TestNaiveList()
         [Test]
         public void TestVirtualisedList()
         {
+            ExampleVirtualisedList list = null!;
             AddStep("create list", () =>
             {
-                ExampleVirtualisedList list;
                 Child = list = new ExampleVirtualisedList
                 {
                     RelativeSizeAxes = Axes.Both,
                 };
                 list.RowData.AddRange(Enumerable.Range(1, 10000).Select(i => $"Item #{i}"));
             });
+            AddStep("replace items", () =>
+            {
+                list.RowData.Clear();
+                list.RowData.AddRange(Enumerable.Range(10001, 10000).Select(i => $"Item #{i}"));
+            });
         }
 
         [Test]

osu.Framework/Graphics/Containers/VirtualisedListContainer.cs

@@ -250,6 +250,19 @@ public void Unload()
                 Visible = false;
                 LifetimeEnd = double.NegativeInfinity;
             }
+
+            protected override void Dispose(bool isDisposing)
+            {
+                // CompositeDrawable only disposes the child pooled drawable (if any).
+                // This is generally not what we want for two reasons:
+                // - We want to try to return the pooled drawable to the pool so that it can be reused.
+                //   Disposing it obviously makes reuse impossible.
+                // - Secondly, pooled drawables return to pool when they lose their parent.
+                // Thus, we proactively clear the pool drawable from ourselves here.
+                Unload();
+
+                base.Dispose(isDisposing);
+            }
         }
 
         protected abstract ScrollContainer<Drawable> CreateScrollContainer();