Preserve modal bottom sheet during interrupted dismiss

Keep the sheet measured from its current visible offset when an entering sheet is dismissed before settling, and let the explicit dismiss path own modal teardown until the hide animation completes.

Author: alexstyl

composeunstyled-bottom-sheet/src/commonMain/kotlin/com/composeunstyled/BottomSheet.kt

@@ -321,12 +321,13 @@ class BottomSheetState(
     } else {
       val currentHeight = anchoredHeightFor(currentDetent)
       val targetHeight = anchoredHeightFor(targetDetent)
+      val offsetHeight = currentOffsetHeight()
 
       when {
-        currentHeight.isNaN() && targetHeight.isNaN() -> maxDetentHeightPx
-        currentHeight.isNaN() -> targetHeight
-        targetHeight.isNaN() -> currentHeight
-        else -> max(currentHeight, targetHeight)
+        currentHeight.isNaN() && targetHeight.isNaN() && offsetHeight.isNaN() -> maxDetentHeightPx
+        else -> listOf(currentHeight, targetHeight, offsetHeight)
+          .filterNot { it.isNaN() }
+          .maxOrNull() ?: maxDetentHeightPx
       }
     }
   }
@@ -336,8 +337,11 @@ class BottomSheetState(
 
     val currentHeight = detentHeightPx(currentDetent, measuredContentHeightPx)
     val targetHeight = detentHeightPx(targetDetent, measuredContentHeightPx)
+    val offsetHeight = currentOffsetHeight().coerceAtMost(measuredContentHeightPx.coerceAtLeast(0f))
 
-    return max(currentHeight, targetHeight)
+    return listOf(currentHeight, targetHeight, offsetHeight)
+      .filterNot { it.isNaN() }
+      .maxOrNull() ?: Float.NaN
   }
 
   internal fun hasContentDependentDetents(): Boolean {
@@ -358,6 +362,15 @@ class BottomSheetState(
     }
   }
 
+  private fun currentOffsetHeight(): Float {
+    val offset = anchoredDraggableState.offset
+    return if (containerHeightPx.isNaN() || offset.isNaN()) {
+      Float.NaN
+    } else {
+      (containerHeightPx - offset).coerceAtLeast(0f)
+    }
+  }
+
   private fun detentHeightPx(detent: SheetDetent, sheetHeightPx: Float): Float {
     if (containerHeightPx.isNaN()) return Float.NaN
 

composeunstyled-modal-bottom-sheet/src/commonMain/kotlin/com/composeunstyled/ModalBottomSheet.kt

@@ -103,6 +103,7 @@ class ModalBottomSheetState internal constructor(
   internal var pendingDetentChange: Job? = null
   internal var modalDetent by mutableStateOf(bottomSheetState.currentDetent)
   internal var dismissAnimationSpec by mutableStateOf(dismissAnimationSpec)
+  internal var dismissAnimationInProgress by mutableStateOf(false)
   private var pendingTargetDetent: SheetDetent? by mutableStateOf(null)
 
   val currentDetent: SheetDetent
@@ -238,10 +239,15 @@ fun UnstyledModalBottomSheet(
 
   fun requestDismiss() {
     dispatchDismiss()
+    state.dismissAnimationInProgress = true
     state.pendingDetentChange?.cancel()
     state.launchPendingDetentChange {
-      state.bottomSheetState.animateTo(SheetDetent.Hidden, state.dismissAnimationSpec)
-      completeDismiss(notifyDismiss = false)
+      try {
+        state.bottomSheetState.animateTo(SheetDetent.Hidden, state.dismissAnimationSpec)
+        completeDismiss(notifyDismiss = false)
+      } finally {
+        state.dismissAnimationInProgress = false
+      }
     }
   }
 
@@ -255,8 +261,13 @@ fun UnstyledModalBottomSheet(
       LaunchedEffect(
         state.bottomSheetState.isIdle,
         state.bottomSheetState.currentDetent,
+        state.dismissAnimationInProgress,
       ) {
-        if (state.bottomSheetState.isIdle && state.bottomSheetState.currentDetent == SheetDetent.Hidden) {
+        if (
+          state.dismissAnimationInProgress.not() &&
+          state.bottomSheetState.isIdle &&
+          state.bottomSheetState.currentDetent == SheetDetent.Hidden
+        ) {
           completeDismiss()
         }
       }

composeunstyled-modal-bottom-sheet/src/commonTest/kotlin/ModalBottomSheet.test.kt

@@ -43,6 +43,8 @@ import assertk.assertThat
 import assertk.assertions.isCloseTo
 import assertk.assertions.isEqualTo
 import assertk.assertions.isFalse
+import assertk.assertions.isGreaterThan
+import assertk.assertions.isLessThan
 import assertk.assertions.isTrue
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.launch
@@ -342,6 +344,49 @@ class ModalBottomSheetTest {
       mainClock.autoAdvance = true
     }
 
+    testCase("modal sheet remains visible while outside tap interrupts enter animation") {
+      val peek = SheetDetent("peek") { containerHeight, _ ->
+        containerHeight * 0.6f
+      }
+      lateinit var state: ModalBottomSheetState
+      setContent {
+        state = rememberModalBottomSheetState(
+          initialDetent = SheetDetent.Hidden,
+          detents = listOf(SheetDetent.Hidden, peek, SheetDetent.FullyExpanded),
+          animationSpec = tween(durationMillis = 1000),
+          dismissAnimationSpec = tween(durationMillis = 1000),
+        )
+        UnstyledModalBottomSheet(
+          state = state,
+          properties = ModalSheetProperties(dismissOnClickOutside = true),
+          overlay = { UnstyledScrim(Modifier.testTag("scrim")) },
+        ) {
+          Sheet { Box(Modifier.testTag("sheet").size(400.dp)) }
+        }
+      }
+      waitForIdle()
+      onNode(isDialog()).assertDoesNotExist()
+
+      mainClock.autoAdvance = false
+      state.targetDetent = peek
+      mainClock.advanceTimeBy(300)
+      onNode(isDialog()).assertExists()
+      onNodeWithTag("sheet").assertExists()
+      assertThat(state.offset).isGreaterThan(0f)
+
+      onNode(isDialog()).performTouchInput { click(Offset(1f, 1f)) }
+      mainClock.advanceTimeByFrame()
+
+      val dialogBottomAfterDismiss = onNode(isDialog()).fetchSemanticsNode().boundsInRoot.bottom
+      val sheetTopAfterDismiss = onNodeWithTag("sheet").fetchSemanticsNode().boundsInRoot.top
+
+      assertThat(state.offset).isGreaterThan(0f)
+      assertThat(sheetTopAfterDismiss).isLessThan(dialogBottomAfterDismiss)
+      onNode(isDialog()).assertExists()
+      onNodeWithTag("sheet").assertExists()
+      mainClock.autoAdvance = true
+    }
+
     testCase(
       "modal sheet unmounts after outside tap dismiss animation and modal fragment exit complete",
     ) {