Step 1: SnackBarBehavior.floating offset fix - Soft breaking change (#50597)

* Adds an opt-in flag to fix floating snackbar's offset when no floating action button is present. This flag will be removed once the migration for the fix is complete.
Co-authored-by: filaps <filip1997.28@mail.ru>

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

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// TODO(shihaohong): remove ignoring deprecated member use analysis
+// when Scaffold.shouldSnackBarIgnoreFABRect parameter is removed.
+// ignore_for_file: deprecated_member_use_from_same_package
+
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
@@ -547,9 +551,19 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
       if (snackBarSize == Size.zero) {
         snackBarSize = layoutChild(_ScaffoldSlot.snackBar, fullWidthConstraints);
       }
-      final double snackBarYOffsetBase = floatingActionButtonRect != null && isSnackBarFloating
+
+      double snackBarYOffsetBase;
+      if (Scaffold.shouldSnackBarIgnoreFABRect) {
+        if (floatingActionButtonRect.size != Size.zero && isSnackBarFloating)
+          snackBarYOffsetBase = floatingActionButtonRect.top;
+        else
+          snackBarYOffsetBase = contentBottom;
+      } else {
+        snackBarYOffsetBase = floatingActionButtonRect != null && isSnackBarFloating
           ? floatingActionButtonRect.top
           : contentBottom;
+      }
+
       positionChild(_ScaffoldSlot.snackBar, Offset(0.0, snackBarYOffsetBase - snackBarSize.height));
     }
 
@@ -1299,6 +1313,17 @@ class Scaffold extends StatefulWidget {
   /// 20.0 will be added to `MediaQuery.of(context).padding.left`.
   final double drawerEdgeDragWidth;
 
+  /// This flag is deprecated and fixes and issue with incorrect clipping
+  /// and positioning of the [SnackBar] set to [SnackBarBehavior.floating].
+  @Deprecated(
+    'This property controls whether to clip and position the snackbar as '
+    'if there is always a floating action button, even if one is not present. '
+    'It exists to provide backwards compatibility to ease migrations, and will '
+    'eventually be removed. '
+    'This feature was deprecated after v1.15.3.'
+  )
+  static bool shouldSnackBarIgnoreFABRect = false;
+
   /// The state from the closest instance of this class that encloses the given context.
   ///
   /// {@tool dartpad --template=freeform}