Don't seek stack animation transition on back start when predictive back animatable is specified

extensions-compose-experimental/src/commonMain/kotlin/com/arkivanov/decompose/extensions/compose/experimental/stack/animation/DefaultStackAnimation.kt

@@ -264,7 +264,7 @@ internal class DefaultStackAnimation<C : Any, T : Any>(
             )
 
             scope.launch {
-                animationHandler.start(backEvent)
+                animationHandler.progress(backEvent)
             }
         }
 
@@ -301,14 +301,6 @@ internal class DefaultStackAnimation<C : Any, T : Any>(
         val exitTransitionState: SeekableTransitionState<EnterExitState> = SeekableTransitionState(EnterExitState.Visible)
         val enterTransitionState: SeekableTransitionState<EnterExitState> = SeekableTransitionState(EnterExitState.PreEnter)
 
-        suspend fun start(backEvent: BackEvent) {
-            awaitAll(
-                { exitTransitionState.seekTo(fraction = backEvent.progress, targetState = EnterExitState.PostExit) },
-                { enterTransitionState.seekTo(fraction = backEvent.progress, targetState = EnterExitState.Visible) },
-                { animatable?.animate(backEvent) },
-            )
-        }
-
         suspend fun progress(backEvent: BackEvent) {
             animatable?.run {
                 animate(backEvent)

extensions-compose-experimental/src/jvmTest/kotlin/com/arkivanov/decompose/extensions/compose/experimental/TestUtils.kt

@@ -1,12 +1,29 @@
 package com.arkivanov.decompose.extensions.compose.experimental
 
+import androidx.compose.animation.EnterExitState
+import androidx.compose.animation.core.LinearEasing
+import androidx.compose.animation.core.Transition
+import androidx.compose.animation.core.animateFloat
+import androidx.compose.animation.core.tween
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.State
 import androidx.compose.ui.semantics.SemanticsConfiguration
 import androidx.compose.ui.semantics.SemanticsNode
 import androidx.compose.ui.semantics.SemanticsProperties
 import androidx.compose.ui.semantics.getOrNull
 import androidx.compose.ui.test.SemanticsNodeInteraction
 import kotlin.test.fail
 
+@Composable
+internal fun Transition<EnterExitState>.animateFloat(): State<Float> =
+    animateFloat(transitionSpec = { tween(easing = LinearEasing) }) { state ->
+        when (state) {
+            EnterExitState.PreEnter -> 0F
+            EnterExitState.Visible -> 1F
+            EnterExitState.PostExit -> 0F
+        }
+    }
+
 internal fun SemanticsNodeInteraction.assertTestTagToRootExists(testTag: String) {
     val count = collectTestTagsToRoot().filter { it == testTag }.size
     if (count != 1) {

extensions-compose-experimental/src/jvmTest/kotlin/com/arkivanov/decompose/extensions/compose/experimental/stack/animation/PredictiveBackGestureTest.kt

@@ -12,6 +12,7 @@ import androidx.compose.ui.test.junit4.createComposeRule
 import androidx.compose.ui.test.onNodeWithText
 import com.arkivanov.decompose.Child
 import com.arkivanov.decompose.ExperimentalDecomposeApi
+import com.arkivanov.decompose.extensions.compose.experimental.animateFloat
 import com.arkivanov.decompose.extensions.compose.experimental.assertTestTagToRootDoesNotExist
 import com.arkivanov.decompose.extensions.compose.experimental.assertTestTagToRootExists
 import com.arkivanov.decompose.extensions.compose.experimental.stack.dropLast
@@ -408,6 +409,66 @@ class PredictiveBackGestureTest {
         assertFalse(values.any { it < 1F })
     }
 
+    @Test
+    fun GIVEN_predictive_animatable_not_null_and_two_children_in_stack_WHEN_gesture_progressed_THEN_animation_transition_not_animating() {
+        var stack by mutableStateOf(stack("1", "2"))
+
+        val animation =
+            DefaultStackAnimation(
+                predictiveBackAnimatable = ::TestAnimatable,
+                onBack = { stack = stack.dropLast() },
+            )
+
+        val values = HashMap<String, Float>()
+
+        composeRule.setContent {
+            animation(stack, Modifier) {
+                val value by transition.animateFloat()
+                values[it.configuration] = value
+            }
+        }
+
+        backDispatcher.startPredictiveBack(BackEvent(progress = 0.1F))
+        composeRule.waitForIdle()
+        backDispatcher.progressPredictiveBack(BackEvent(progress = 0.2F))
+        composeRule.waitForIdle()
+        backDispatcher.progressPredictiveBack(BackEvent(progress = 0.3F))
+        composeRule.waitForIdle()
+
+        assertEquals(0F, values["1"])
+        assertEquals(1F, values["2"])
+    }
+
+    @Test
+    fun GIVEN_predictive_animatable_null_and_two_children_in_stack_WHEN_gesture_progressed_THEN_animation_transition_animating() {
+        var stack by mutableStateOf(stack("1", "2"))
+
+        val animation =
+            DefaultStackAnimation(
+                predictiveBackAnimatable = { null },
+                onBack = { stack = stack.dropLast() },
+            )
+
+        val values = HashMap<String, Float>()
+
+        composeRule.setContent {
+            animation(stack, Modifier) {
+                val value by transition.animateFloat()
+                values[it.configuration] = value
+            }
+        }
+
+        backDispatcher.startPredictiveBack(BackEvent(progress = 0.1F))
+        composeRule.waitForIdle()
+        backDispatcher.progressPredictiveBack(BackEvent(progress = 0.2F))
+        composeRule.waitForIdle()
+        backDispatcher.progressPredictiveBack(BackEvent(progress = 0.3F))
+        composeRule.waitForIdle()
+
+        assertEquals(0.3F, values["1"])
+        assertEquals(0.7F, values["2"])
+    }
+
     private fun DefaultStackAnimation(
         predictiveBackAnimatable: (initialBackEvent: BackEvent) -> PredictiveBackAnimatable? = ::TestAnimatable,
         onBack: () -> Unit,