Fix floating SnackBar throws when FAB is on the top (#129274)

## Description

This PR updates how a floating snack bar is positionned when a `Scaffold` defines a FAB with `Scaffold.floatingActionButtonLocation` sets to one of the top locations.

**Before this PR:**
- When a FAB location is set to the top of the `Scaffold`, a floating `SnackBar` can't be displayed and an assert throws in debug mode.

**After this PR:**
- When a FAB location is set to the top of the `Scaffold`, a floating `SnackBar` will be displayed at the bottom of the screen, above a `NavigationBar` for instance (the top FAB is ignored when computing the floating snack bar position).

![image](https://github.com/flutter/flutter/assets/840911/08fcee6c-b286-4749-ad0b-ba09e653bd94)

## Motivation

This is a edge case related to a discrepancy between the Material spec and the Flutter `Scaffold` customizability:
- Material spec states that a floating `SnackBar` should be displayed above a FAB. But, in Material spec, FABs are expected to be on the bottom.
- Since https://github.com/flutter/flutter/issues/51465, Flutter `Scaffold` makes it valid to show a FAB on the top of the `Scaffold`.

## Related Issue

fixes https://github.com/flutter/flutter/issues/128150

## Tests

Adds 1 test.

packages/flutter/lib/src/material/scaffold.dart

@@ -1142,7 +1142,31 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
       }
 
       final double snackBarYOffsetBase;
-      if (floatingActionButtonRect.size != Size.zero && isSnackBarFloating) {
+      final bool showAboveFab = switch (currentFloatingActionButtonLocation) {
+        FloatingActionButtonLocation.startTop
+        || FloatingActionButtonLocation.centerTop
+        || FloatingActionButtonLocation.endTop
+        || FloatingActionButtonLocation.miniStartTop
+        || FloatingActionButtonLocation.miniCenterTop
+        || FloatingActionButtonLocation.miniEndTop => false,
+        FloatingActionButtonLocation.startDocked
+        || FloatingActionButtonLocation.startFloat
+        || FloatingActionButtonLocation.centerDocked
+        || FloatingActionButtonLocation.centerFloat
+        || FloatingActionButtonLocation.endContained
+        || FloatingActionButtonLocation.endDocked
+        || FloatingActionButtonLocation.endFloat
+        || FloatingActionButtonLocation.miniStartDocked
+        || FloatingActionButtonLocation.miniStartFloat
+        || FloatingActionButtonLocation.miniCenterDocked
+        || FloatingActionButtonLocation.miniCenterFloat
+        || FloatingActionButtonLocation.miniEndDocked
+        || FloatingActionButtonLocation.miniEndFloat => true,
+        FloatingActionButtonLocation() => throw FlutterError(
+          '$currentFloatingActionButtonLocation is an unknown FloatingActionButtonLocation value.'
+        ),
+      };
+      if (floatingActionButtonRect.size != Size.zero && isSnackBarFloating && showAboveFab) {
         snackBarYOffsetBase = floatingActionButtonRect.top;
       } else {
         // SnackBarBehavior.fixed applies a SafeArea automatically.

packages/flutter/lib/src/material/snack_bar_theme.dart

@@ -12,19 +12,22 @@ import 'theme.dart';
 
 /// Defines where a [SnackBar] should appear within a [Scaffold] and how its
 /// location should be adjusted when the scaffold also includes a
-/// [FloatingActionButton] or a [BottomNavigationBar].
+/// [FloatingActionButton], a [BottomNavigationBar], or a [NavigationBar].
 enum SnackBarBehavior {
   /// Fixes the [SnackBar] at the bottom of the [Scaffold].
   ///
   /// The exception is that the [SnackBar] will be shown above a
-  /// [BottomNavigationBar]. Additionally, the [SnackBar] will cause other
+  /// [BottomNavigationBar] or a [NavigationBar]. Additionally, the [SnackBar]
-  /// non-fixed widgets inside [Scaffold] to be pushed above (for example, the
+  /// will cause other non-fixed widgets inside [Scaffold] to be pushed above
-  /// [FloatingActionButton]).
+  /// (for example, the [FloatingActionButton]).
   fixed,
 
   /// This behavior will cause [SnackBar] to be shown above other widgets in the
-  /// [Scaffold]. This includes being displayed above a [BottomNavigationBar]
+  /// [Scaffold]. This includes being displayed above a [BottomNavigationBar] or
-  /// and a [FloatingActionButton].
+  /// a [NavigationBar], and a [FloatingActionButton] when its location is on the
+  /// bottom. When the floating action button location is on the top, this behavior
+  /// will cause the [SnackBar] to be shown above other widgets in the [Scaffold]
+  /// except the floating action button.
   ///
   /// See <https://material.io/design/components/snackbars.html> for more details.
   floating,

packages/flutter/test/material/snack_bar_test.dart

@@ -2071,6 +2071,59 @@ void main() {
       },
     );
 
+    testWidgets(
+      '${SnackBarBehavior.floating} should not align SnackBar with the top of FloatingActionButton '
+      'when Scaffold has a FloatingActionButton and floatingActionButtonLocation is set to a top position',
+      (WidgetTester tester) async {
+        Future<void> pumpApp({required FloatingActionButtonLocation fabLocation}) async {
+          return tester.pumpWidget(MaterialApp(
+            home: Scaffold(
+              floatingActionButton: FloatingActionButton(
+                child: const Icon(Icons.send),
+                onPressed: () {},
+              ),
+              floatingActionButtonLocation: fabLocation,
+              body: Builder(
+                builder: (BuildContext context) {
+                  return GestureDetector(
+                    onTap: () {
+                      ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                        content: const Text('I am a snack bar.'),
+                        duration: const Duration(seconds: 2),
+                        action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+                        behavior: SnackBarBehavior.floating,
+                      ));
+                    },
+                    child: const Text('X'),
+                  );
+                },
+              ),
+            ),
+          ));
+        }
+
+        const List<FloatingActionButtonLocation> topLocations = <FloatingActionButtonLocation>[
+          FloatingActionButtonLocation.startTop,
+          FloatingActionButtonLocation.centerTop,
+          FloatingActionButtonLocation.endTop,
+          FloatingActionButtonLocation.miniStartTop,
+          FloatingActionButtonLocation.miniCenterTop,
+          FloatingActionButtonLocation.miniEndTop,
+        ];
+
+        for (final FloatingActionButtonLocation location in topLocations) {
+          await pumpApp(fabLocation: location);
+
+          await tester.tap(find.text('X'));
+          await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
+
+          final Offset snackBarBottomLeft = tester.getBottomLeft(find.byType(SnackBar));
+
+          expect(snackBarBottomLeft.dy, 600); // Device height is 600.
+        }
+      },
+    );
+
     testWidgets(
       '${SnackBarBehavior.fixed} should align SnackBar with the top of BottomNavigationBar '
       'when Scaffold has a BottomNavigationBar and FloatingActionButton',