Fix a host crash in LazyListUpdateProcessor (#2177)

I introduced a bug in the mechanism that attempts to replace
animated inserts and removes with update-in-place transitions.

The crash occurs when the before window shrinks while the total
number of elements is growing. The previous code naively
assumed the before window shrinking would always be a scroll,
but it can coincide with a content change.

Closes: #2172

CHANGELOG.md

@@ -14,6 +14,7 @@ Changed:
 Fixed:
 - Using a `data object` for a widget of modifier no longer causes schema parsing to crash.
 - Ensuring `LazyList`'s `itemsBefore` and `itemsAfter` properties are always within `[0, itemCount]`, to prevent `IndexOutOfBoundsException` crashes.
+- Don't crash in `LazyList` when a scroll and content change occur in the same update.
 - Updating a flex container's margin now works correctly for Yoga-based layouts.
 
 Upgraded:

redwood-lazylayout-widget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/LazyListUpdateProcessor.kt

@@ -307,7 +307,7 @@ public abstract class LazyListUpdateProcessor<V : Any, W : Any> {
       itemsAfter.addRange(0, itemsBefore, splitIndex, count)
       itemsBefore.removeRange(splitIndex, splitIndex + count)
     } else if (splitIndex > itemsBefore.size) {
-      val count = splitIndex - itemsBefore.size
+      val count = minOf(splitIndex - itemsBefore.size, itemsAfter.size)
       itemsBefore.addRange(itemsBefore.size, itemsAfter, 0, count)
       itemsAfter.removeRange(0, count)
     }

redwood-lazylayout-widget/src/commonTest/kotlin/app/cash/redwood/lazylayout/widget/LazyListUpdateProcessorTest.kt

@@ -566,6 +566,89 @@ class LazyListUpdateProcessorTest {
     assertThat(processor.toString()).isEqualTo("[2...] . . . . [...13]")
   }
 
+  /**
+   * We had a bug where we attempted to move the split between `itemsBefore` and `itemsAfter` beyond
+   * the end of `itemsAfter`. This bug would occurs when the following held:
+   *
+   *  * The items before count is shrinking
+   *  * The insert size is greater than all unloaded items (itemsBefore + itemsAfter)
+   *
+   * https://github.com/cashapp/redwood/issues/2172
+   */
+  @Test
+  fun shiftItemsFromAfterToBeforeWhenThereAreNotEnoughToShift() {
+    processor.itemsBefore(3)
+    processor.itemsAfter(3)
+    processor.onEndChanges()
+    processor.scrollTo(0, 6)
+    assertThat(processor.toString()).isEqualTo(". . . . . .")
+
+    processor.itemsBefore(2)
+    processor.items.insert(0, "C")
+    processor.items.insert(1, "D")
+    processor.items.insert(2, "E")
+    processor.items.insert(3, "F")
+    processor.items.insert(4, "G")
+    processor.items.insert(5, "H")
+    processor.items.insert(6, "I")
+    processor.onEndChanges()
+    processor.scrollTo(0, 12)
+    assertThat(processor.toString()).isEqualTo(". . C D E Fv2 G H I . . .")
+  }
+
+  @Test
+  fun shiftItemsFromAfterToBeforeWhenAfterIsEmpty() {
+    processor.itemsBefore(1)
+    processor.itemsAfter(0)
+    processor.onEndChanges()
+    processor.scrollTo(0, 1)
+    assertThat(processor.toString()).isEqualTo(".")
+
+    processor.itemsBefore(0)
+    processor.items.insert(0, "A")
+    processor.items.insert(1, "B")
+    processor.onEndChanges()
+    processor.scrollTo(0, 2)
+    assertThat(processor.toString()).isEqualTo("A B")
+  }
+
+  @Test
+  fun shiftItemsFromBeforeToAfterWhenThereAreNotEnoughToShift() {
+    processor.itemsBefore(3)
+    processor.itemsAfter(3)
+    processor.onEndChanges()
+    processor.scrollTo(0, 6)
+    assertThat(processor.toString()).isEqualTo(". . . . . .")
+
+    processor.items.insert(0, "C")
+    processor.items.insert(1, "D")
+    processor.items.insert(2, "E")
+    processor.items.insert(3, "F")
+    processor.items.insert(4, "G")
+    processor.items.insert(5, "H")
+    processor.items.insert(6, "I")
+    processor.itemsAfter(2)
+    processor.onEndChanges()
+    processor.scrollTo(0, 12)
+    assertThat(processor.toString()).isEqualTo(". . . Cv2 D E F G H I . .")
+  }
+
+  @Test
+  fun shiftItemsFromBeforeToAfterWhenBeforeIsEmpty() {
+    processor.itemsBefore(0)
+    processor.itemsAfter(1)
+    processor.onEndChanges()
+    processor.scrollTo(0, 1)
+    assertThat(processor.toString()).isEqualTo(".")
+
+    processor.itemsAfter(0)
+    processor.items.insert(0, "A")
+    processor.items.insert(1, "B")
+    processor.onEndChanges()
+    processor.scrollTo(0, 2)
+    assertThat(processor.toString()).isEqualTo("Av2 B")
+  }
+
   private fun Widget.Children<StringContent>.insert(index: Int, value: String) {
     val content = StringContent(value)
     val widget = object : Widget<StringContent> {