diff --git a/redwood-lazylayout-compose/src/commonTest/kotlin/app/cash/redwood/lazylayout/compose/LazyListTest.kt b/redwood-lazylayout-compose/src/commonTest/kotlin/app/cash/redwood/lazylayout/compose/LazyListTest.kt
index ce7376a273..f0a1b6309f 100644
--- a/redwood-lazylayout-compose/src/commonTest/kotlin/app/cash/redwood/lazylayout/compose/LazyListTest.kt
+++ b/redwood-lazylayout-compose/src/commonTest/kotlin/app/cash/redwood/lazylayout/compose/LazyListTest.kt
@@ -16,19 +16,31 @@
 package app.cash.redwood.lazylayout.compose
 
 import app.cash.redwood.Modifier
+import app.cash.redwood.RedwoodCodegenApi
 import app.cash.redwood.layout.api.Constraint
 import app.cash.redwood.layout.api.CrossAxisAlignment
+import app.cash.redwood.layout.widget.RedwoodLayoutTestingWidgetFactory
 import app.cash.redwood.lazylayout.api.ScrollItemIndex
 import app.cash.redwood.lazylayout.widget.LazyListValue
+import app.cash.redwood.lazylayout.widget.ListUpdateCallback
+import app.cash.redwood.lazylayout.widget.RedwoodLazyLayoutTestingWidgetFactory
+import app.cash.redwood.lazylayout.widget.RedwoodLazyLayoutWidgetFactory
+import app.cash.redwood.lazylayout.widget.WindowedLazyList
+import app.cash.redwood.testing.TestRedwoodComposition
 import app.cash.redwood.testing.WidgetValue
 import app.cash.redwood.ui.Margin
+import app.cash.redwood.ui.UiConfiguration
 import app.cash.redwood.ui.dp
+import app.cash.redwood.widget.MutableListChildren
 import assertk.assertThat
 import assertk.assertions.containsExactly
 import com.example.redwood.testing.compose.Text
-import com.example.redwood.testing.widget.TestSchemaTester
+import com.example.redwood.testing.widget.TestSchemaTestingWidgetFactory
+import com.example.redwood.testing.widget.TestSchemaWidgetFactories
 import com.example.redwood.testing.widget.TextValue
 import kotlin.test.Test
+import kotlinx.coroutines.coroutineScope
+import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.test.runTest
 
 class LazyListTest {
@@ -86,6 +98,136 @@ class LazyListTest {
         ),
       )
   }
+
+  @OptIn(RedwoodCodegenApi::class)
+  @Test
+  fun scrollPopulatedLazyColumn() = runTest {
+    val windowedLazyList = FakeWindowedLazyList()
+    val factories = TestSchemaWidgetFactories(
+      TestSchema = TestSchemaTestingWidgetFactory(),
+      RedwoodLayout = RedwoodLayoutTestingWidgetFactory(),
+      RedwoodLazyLayout = RedwoodLazyLayoutFakeWidgetFactory(windowedLazyList),
+    )
+    val snapshot = TestSchemaTester(factories) {
+      setContent {
+        LazyColumn(placeholder = { Text("Placeholder") }) {
+          items(100) {
+            Text(it.toString())
+          }
+        }
+      }
+      awaitSnapshot()
+      windowedLazyList.updateViewport(50, 60)
+      awaitSnapshot()
+    }
+
+    assertThat(snapshot)
+      .containsExactly(
+        DefaultLazyListValue.copy(
+          itemsBefore = 35,
+          itemsAfter = 25,
+          placeholder = List(20) { TextValue(Modifier, "Placeholder") },
+          items = List(40) { TextValue(Modifier, (it + 35).toString()) },
+        ),
+      )
+  }
+}
+
+@OptIn(RedwoodCodegenApi::class)
+private suspend fun <R> TestSchemaTester(
+  factories: TestSchemaWidgetFactories<WidgetValue> = TestSchemaWidgetFactories(
+    TestSchema = TestSchemaTestingWidgetFactory(),
+    RedwoodLayout = RedwoodLayoutTestingWidgetFactory(),
+    RedwoodLazyLayout = RedwoodLazyLayoutTestingWidgetFactory(),
+  ),
+  body: suspend TestRedwoodComposition<List<WidgetValue>>.() -> R,
+): R =
+  coroutineScope {
+    val container = MutableListChildren<WidgetValue>()
+    val tester = TestRedwoodComposition(
+      this,
+      factories,
+      container,
+      MutableStateFlow(UiConfiguration()),
+    ) {
+      container.map { it.value }
+    }
+    try {
+      tester.body()
+    } finally {
+      tester.cancel()
+    }
+  }
+
+private class RedwoodLazyLayoutFakeWidgetFactory(
+  private val windowedLazyList: WindowedLazyList<WidgetValue>,
+) : RedwoodLazyLayoutWidgetFactory<WidgetValue> {
+  override fun LazyList() = windowedLazyList
+  override fun RefreshableLazyList() = error("unexpected call")
+}
+
+private class FakeWindowedLazyList : WindowedLazyList<WidgetValue>(NoOpListUpdateCallback) {
+  override val `value`: WidgetValue
+    get() = LazyListValue(
+      modifier = modifier,
+      isVertical = isVertical!!,
+      onViewportChanged = onViewportChanged!!,
+      itemsBefore = items.itemsBefore,
+      itemsAfter = items.itemsAfter,
+      width = width!!,
+      height = height!!,
+      margin = margin!!,
+      crossAxisAlignment = crossAxisAlignment!!,
+      scrollItemIndex = scrollItemIndex!!,
+      placeholder = placeholder.map { it.`value` },
+      items = items.filterNotNull().map { it.`value` },
+    )
+
+  override var modifier: Modifier = Modifier
+
+  private var isVertical: Boolean? = null
+
+  private var width: Constraint? = null
+
+  private var height: Constraint? = null
+
+  private var margin: Margin? = null
+
+  private var crossAxisAlignment: CrossAxisAlignment? = null
+
+  private var scrollItemIndex: ScrollItemIndex? = null
+
+  override val placeholder: MutableListChildren<WidgetValue> = MutableListChildren()
+
+  override fun isVertical(isVertical: Boolean) {
+    this.isVertical = isVertical
+  }
+
+  override fun width(width: Constraint) {
+    this.width = width
+  }
+
+  override fun height(height: Constraint) {
+    this.height = height
+  }
+
+  override fun margin(margin: Margin) {
+    this.margin = margin
+  }
+
+  override fun crossAxisAlignment(crossAxisAlignment: CrossAxisAlignment) {
+    this.crossAxisAlignment = crossAxisAlignment
+  }
+
+  override fun scrollItemIndex(scrollItemIndex: ScrollItemIndex) {
+    this.scrollItemIndex = scrollItemIndex
+  }
+}
+
+private object NoOpListUpdateCallback : ListUpdateCallback {
+  override fun onInserted(position: Int, count: Int) = Unit
+  override fun onMoved(fromPosition: Int, toPosition: Int, count: Int) = Unit
+  override fun onRemoved(position: Int, count: Int) = Unit
 }
 
 private val DefaultLazyListValue = LazyListValue(
diff --git a/redwood-lazylayout-widget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/WindowedLazyList.kt b/redwood-lazylayout-widget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/WindowedLazyList.kt
index 163e4b5f90..4036f1c383 100644
--- a/redwood-lazylayout-widget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/WindowedLazyList.kt
+++ b/redwood-lazylayout-widget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/WindowedLazyList.kt
@@ -21,7 +21,7 @@ public abstract class WindowedLazyList<W : Any>(
 
   private var firstVisibleItemIndex = 0
   private var lastVisibleItemIndex = 0
-  private var onViewportChanged: ((firstVisibleItemIndex: Int, lastVisibleItemIndex: Int) -> Unit)? = null
+  protected var onViewportChanged: ((firstVisibleItemIndex: Int, lastVisibleItemIndex: Int) -> Unit)? = null
 
   final override val items: WindowedChildren<W> = WindowedChildren(listUpdateCallback)
 
@@ -29,7 +29,7 @@ public abstract class WindowedLazyList<W : Any>(
     this.onViewportChanged = onViewportChanged
   }
 
-  protected fun updateViewport(firstVisibleItemIndex: Int, lastVisibleItemIndex: Int) {
+  public fun updateViewport(firstVisibleItemIndex: Int, lastVisibleItemIndex: Int) {
     if (firstVisibleItemIndex != this.firstVisibleItemIndex || lastVisibleItemIndex != this.lastVisibleItemIndex) {
       this.firstVisibleItemIndex = firstVisibleItemIndex
       this.lastVisibleItemIndex = lastVisibleItemIndex