Avoid NPE when nested drag starts before enabling nestedDraggable()

This CL fixes an NPE that was happening when a nested drag was started
right before enabling a nestedDraggable(), leading to a null
lastFirstDown.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT NestedDraggable is not used yet
Change-Id: Ib7a4122b2815b1d1e1a664f04bcc5d6bd76bf2d8

2 files changed, 26 insertions, 2 deletions

diff --git a/packages/SystemUI/compose/core/src/com/android/compose/gesture/NestedDraggable.kt b/packages/SystemUI/compose/core/src/com/android/compose/gesture/NestedDraggable.kt
index 96401ce6e1c7..76dda8099e07 100644
--- a/packages/SystemUI/compose/core/src/com/android/compose/gesture/NestedDraggable.kt
+++ b/packages/SystemUI/compose/core/src/com/android/compose/gesture/NestedDraggable.kt
@@ -549,9 +549,10 @@ private class NestedDraggableNode(
             nestedScrollController == null &&
                 // TODO(b/388231324): Remove this.
                 !lastEventWasScrollWheel &&
-                draggable.shouldConsumeNestedScroll(sign)
+                draggable.shouldConsumeNestedScroll(sign) &&
+                lastFirstDown != null
         ) {
-            val startedPosition = checkNotNull(lastFirstDown) { "lastFirstDown is not set" }
+            val startedPosition = checkNotNull(lastFirstDown)
 
             // TODO(b/382665591): Ensure that there is at least one pointer down.
             val pointersDownCount = pointersDown.size.coerceAtLeast(1)
diff --git a/packages/SystemUI/compose/core/tests/src/com/android/compose/gesture/NestedDraggableTest.kt b/packages/SystemUI/compose/core/tests/src/com/android/compose/gesture/NestedDraggableTest.kt
index 5de0f1221f0f..8fa8e825b9ab 100644
--- a/packages/SystemUI/compose/core/tests/src/com/android/compose/gesture/NestedDraggableTest.kt
+++ b/packages/SystemUI/compose/core/tests/src/com/android/compose/gesture/NestedDraggableTest.kt
@@ -773,6 +773,29 @@ class NestedDraggableTest(override val orientation: Orientation) : OrientationAw
         rule.onNodeWithTag(buttonTag).assertTextEquals("Count: 3")
     }
 
+    @Test
+    fun nestedDragNotStartedWhenEnabledAfterDragStarted() {
+        val draggable = TestDraggable()
+        var enabled by mutableStateOf(false)
+        val touchSlop =
+            rule.setContentWithTouchSlop {
+                Box(
+                    Modifier.fillMaxSize()
+                        .nestedDraggable(draggable, orientation, enabled = enabled)
+                        .scrollable(rememberScrollableState { 0f }, orientation)
+                )
+            }
+
+        rule.onRoot().performTouchInput { down(center) }
+
+        enabled = true
+        rule.waitForIdle()
+
+        rule.onRoot().performTouchInput { moveBy((touchSlop + 1f).toOffset()) }
+
+        assertThat(draggable.onDragStartedCalled).isFalse()
+    }
+
     private fun ComposeContentTestRule.setContentWithTouchSlop(
         content: @Composable () -> Unit
     ): Float {