Use fade animation when replacing Placeholder

Author: ganfra

appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowNode.kt

@@ -636,7 +636,7 @@ class LoggedInFlowNode(
             ) { contentModifier ->
                 Box(modifier = contentModifier) {
                     val ftueState by ftueService.state.collectAsState()
-                    BackstackView()
+                    BackstackView(transitionHandler = rememberLoggedInFlowTransitionHandler(backstack))
                     if (ftueState is FtueState.Complete) {
                         PermanentChild(permanentNavModel = permanentNavModel, navTarget = NavTarget.LoggedInPermanent)
                     }

appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowTransitionHandler.kt

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.appnav
+
+import androidx.compose.animation.core.Spring
+import androidx.compose.animation.core.Transition
+import androidx.compose.animation.core.spring
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import com.bumble.appyx.core.navigation.transition.ModifierTransitionHandler
+import com.bumble.appyx.core.navigation.transition.TransitionDescriptor
+import com.bumble.appyx.navmodel.backstack.BackStack
+import com.bumble.appyx.navmodel.backstack.transitionhandler.rememberBackstackFader
+import com.bumble.appyx.navmodel.backstack.transitionhandler.rememberBackstackSlider
+
+/**
+ * A TransitionHandler that uses fade transition when Placeholder is being removed,
+ * and slide transition for all other cases.
+ */
+class LoggedInFlowTransitionHandler(
+    private val backstack: BackStack<LoggedInFlowNode.NavTarget>,
+    private val slider: ModifierTransitionHandler<LoggedInFlowNode.NavTarget, BackStack.State>,
+    private val fader: ModifierTransitionHandler<LoggedInFlowNode.NavTarget, BackStack.State>,
+) : ModifierTransitionHandler<LoggedInFlowNode.NavTarget, BackStack.State>() {
+    override fun createModifier(
+        modifier: Modifier,
+        transition: Transition<BackStack.State>,
+        descriptor: TransitionDescriptor<LoggedInFlowNode.NavTarget, BackStack.State>
+    ): Modifier {
+        val isPlaceholderBeingRemoved = backstack.elements.value.any { element ->
+            element.key.navTarget == LoggedInFlowNode.NavTarget.Placeholder &&
+                element.targetState != BackStack.State.ACTIVE
+        }
+        val handler = if (isPlaceholderBeingRemoved) fader else slider
+        return handler.createModifier(modifier, transition, descriptor)
+    }
+}
+
+@Composable
+fun rememberLoggedInFlowTransitionHandler(
+    backstack: BackStack<LoggedInFlowNode.NavTarget>,
+): ModifierTransitionHandler<LoggedInFlowNode.NavTarget, BackStack.State> {
+    val slider = rememberBackstackSlider<LoggedInFlowNode.NavTarget>(
+        transitionSpec = { spring(stiffness = Spring.StiffnessMediumLow) },
+    )
+    val fader = rememberBackstackFader<LoggedInFlowNode.NavTarget>(
+        transitionSpec = { spring(stiffness = Spring.StiffnessMediumLow) },
+    )
+    return remember(backstack, slider, fader) {
+        LoggedInFlowTransitionHandler(backstack, slider, fader)
+    }
+}