Finish predictive back from the scaled preview

CHANGELOG.md

@@ -12,6 +12,11 @@ are no binary releases; "version" tracks the shared `version` in the root build
 
 ### Fixed
 
+- **`identity`** — A committed predictive back no longer snaps the surface back
+  to full size before it leaves. `Modifier.predictiveBack()` now holds the
+  preview transform after `onBack`, and the new `predictiveBackExit()` exit
+  fades and keeps shrinking from there; pair them on the surface's
+  `AnimatedVisibility` instead of a full-size slide-out.
 - **`core-crash`** — The prefilled crash issue is filed in English again. Its
   title and body came from string resources, so a crash on a French phone
   opened the tracker with a French issue — unreadable to whoever has to triage

identity/src/main/kotlin/de/jeanlucmakiola/floret/identity/Motion.kt

@@ -3,9 +3,12 @@ package de.jeanlucmakiola.floret.identity
 import android.provider.Settings
 import androidx.activity.BackEventCompat
 import androidx.activity.compose.PredictiveBackHandler
+import androidx.compose.animation.ExitTransition
 import androidx.compose.animation.animateContentSize
 import androidx.compose.animation.core.Animatable
 import androidx.compose.animation.core.FastOutSlowInEasing
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.scaleOut
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
 import androidx.compose.material3.MaterialTheme
@@ -19,6 +22,7 @@ import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.IntSize
 import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.delay
 import kotlin.coroutines.cancellation.CancellationException
 
 /**
@@ -59,7 +63,11 @@ fun Modifier.animateContentSizeMotion(reduceMotion: Boolean = rememberReduceMoti
  * The standard Android predictive-back transform for a full-screen surface: as
  * the back gesture is dragged, the surface scales toward ~90%, shifts toward the
  * swiped edge and rounds its corners, previewing what's behind. Completing the
- * gesture invokes [onBack]; cancelling springs it back.
+ * gesture invokes [onBack] and keeps the preview transform, so the surface
+ * leaves from where the finger let go instead of snapping back to full size —
+ * pair it with [predictiveBackExit] on the surface's own exit. Should the surface
+ * still be there shortly after (the back was taken without dismissing it), it
+ * settles back. Cancelling springs it back.
  *
  * A drop-in replacement for a screen's own `BackHandler(onBack)` — register it
  * once and apply the returned [Modifier] to that screen's root so the preview
@@ -87,7 +95,8 @@ fun Modifier.predictiveBack(
                 progress.snapTo(FastOutSlowInEasing.transform(event.progress))
             }
             onBack()
-            progress.snapTo(0f)
+            delay(COMMIT_SETTLE_DELAY_MILLIS)
+            progress.animateTo(0f)
         } catch (_: CancellationException) {
             progress.animateTo(0f)
         }
@@ -104,3 +113,26 @@ fun Modifier.predictiveBack(
         clip = p > 0f
     }
 }
+
+/**
+ * How long a committed preview is held before it settles back — past any exit
+ * the surface would play, so it only fires for a back that left it in place.
+ */
+private const val COMMIT_SETTLE_DELAY_MILLIS = 1_000L
+
+/**
+ * The exit for a surface closed through [predictiveBack]: it fades while it keeps
+ * shrinking from the preview's scale, rather than growing back and sliding off
+ * at full size. Also reads right for a close without a gesture. Fade only under
+ * reduced motion.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun predictiveBackExit(reduceMotion: Boolean = rememberReduceMotion()): ExitTransition {
+    val fade = fadeOut(MaterialTheme.motionScheme.defaultEffectsSpec())
+    return if (reduceMotion) {
+        fade
+    } else {
+        fade + scaleOut(MaterialTheme.motionScheme.defaultSpatialSpec(), targetScale = 0.9f)
+    }
+}