identity: add Modifier.animateContentSizeMotion()

Eases a composable's own height/size changes on the family's M3 Expressive
spatial motion scheme (fastSpatialSpec) instead of jumping — e.g. a text
field that wraps to a new line, or a value swapped for a longer one. Snaps
under reduced motion, like the other identity motion helpers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -30,8 +30,11 @@ are no binary releases; "version" tracks the shared `version` in the root build
   own thin themed `CrashReportActivity`.
 - **`identity`** (Android library) — the family's Material 3 Expressive theme
   factory `FloretExpressiveTheme(lightScheme, darkScheme, …)` (dynamic colour on
-  API 31+, system light/dark fallback, the standard motion scheme) plus
+  API 31+, system light/dark fallback, the standard motion scheme) plus the
-  `rememberNavSlideSpec()`. Each app passes its own seed-derived colour schemes
+  shared motion helpers — `rememberNavSlideSpec()`, the content transitions,
+  `Modifier.predictiveBack()` and `Modifier.animateContentSizeMotion()` (eases a
+  block's own height changes, e.g. a text field wrapping to a new line), each
+  reduced-motion-aware. Each app passes its own seed-derived colour schemes
   and typography — the mechanics are shared, the look is not.
 - **`components`** (Android library) — the shared Compose component vocabulary:
   - `GroupedSurface` / `GroupedRow` + `Position` / `positionOf` — the canonical

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.animateContentSize
 import androidx.compose.animation.core.Animatable
 import androidx.compose.animation.core.FastOutSlowInEasing
 import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
@@ -14,6 +17,7 @@ import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 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 kotlin.coroutines.cancellation.CancellationException
 
@@ -36,6 +40,21 @@ fun rememberReduceMotion(): Boolean {
     }
 }
 
+/**
+ * Animates a composable's own size changes with the family's spatial motion, so
+ * a block that grows or shrinks — a text field wrapping to a new line, a value
+ * swapped for a longer one — eases into its new height instead of jumping. Wraps
+ * Compose's [androidx.compose.animation.animateContentSize] onto the active M3
+ * Expressive motion scheme; under reduced motion the new size snaps in.
+ *
+ * Apply it high in the modifier chain of the element whose height changes.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun Modifier.animateContentSizeMotion(reduceMotion: Boolean = rememberReduceMotion()): Modifier =
+    if (reduceMotion) this
+    else animateContentSize(animationSpec = MaterialTheme.motionScheme.fastSpatialSpec<IntSize>())
+
 /**
  * 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