Improved bottom media padding for SnackBars (#14396)

In a scaffold, snackbars are positioned above the BottomNavigationBar
and/or PersistentBottomButtons, if present. In such cases, they should
not apply bottom media padding to their widget sub-tree.

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

@@ -886,6 +886,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     }
 
     if (_snackBars.isNotEmpty) {
+      final bool removeBottomPadding = widget.persistentFooterButtons != null || widget.bottomNavigationBar != null;
       _addIfNonNull(
         children,
         _snackBars.first._widget,
@@ -893,7 +894,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
         removeLeftPadding: false,
         removeTopPadding: true,
         removeRightPadding: false,
-        removeBottomPadding: false,
+        removeBottomPadding: removeBottomPadding,
       );
     }
 

packages/flutter/test/material/snack_bar_test.dart

@@ -347,6 +347,63 @@ void main() {
     expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 14.0 + 40.0); // margin + bottom padding
   });
 
+  testWidgets('SnackBar is positioned above BottomNavigationBar', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new MediaQuery(
+        data: const MediaQueryData(
+          padding: const EdgeInsets.only(
+            left: 10.0,
+            top: 20.0,
+            right: 30.0,
+            bottom: 40.0,
+          ),
+        ),
+        child: new Scaffold(
+          bottomNavigationBar: new BottomNavigationBar(
+            items: <BottomNavigationBarItem>[
+              const BottomNavigationBarItem(icon: const Icon(Icons.favorite), title: const Text('Animutation')),
+              const BottomNavigationBarItem(icon: const Icon(Icons.block), title: const Text('Zombo.com')),
+            ],
+          ),
+          body: new Builder(
+            builder: (BuildContext context) {
+              return new GestureDetector(
+                onTap: () {
+                  Scaffold.of(context).showSnackBar(new SnackBar(
+                    content: const Text('I am a snack bar.'),
+                    duration: const Duration(seconds: 2),
+                    action: new SnackBarAction(label: 'ACTION', onPressed: () {})
+                  ));
+                },
+                child: const Text('X')
+              );
+            }
+          ),
+        ),
+      ),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    final RenderBox textBox = tester.firstRenderObject(find.text('I am a snack bar.'));
+    final RenderBox actionTextBox = tester.firstRenderObject(find.text('ACTION'));
+    final RenderBox snackBarBox = tester.firstRenderObject(find.byType(SnackBar));
+
+    final Offset textBottomLeft = textBox.localToGlobal(textBox.size.bottomLeft(Offset.zero));
+    final Offset textBottomRight = textBox.localToGlobal(textBox.size.bottomRight(Offset.zero));
+    final Offset actionTextBottomLeft = actionTextBox.localToGlobal(actionTextBox.size.bottomLeft(Offset.zero));
+    final Offset actionTextBottomRight = actionTextBox.localToGlobal(actionTextBox.size.bottomRight(Offset.zero));
+    final Offset snackBarBottomLeft = snackBarBox.localToGlobal(snackBarBox.size.bottomLeft(Offset.zero));
+    final Offset snackBarBottomRight = snackBarBox.localToGlobal(snackBarBox.size.bottomRight(Offset.zero));
+
+    expect(textBottomLeft.dx - snackBarBottomLeft.dx, 24.0 + 10.0); // margin + left padding
+    expect(snackBarBottomLeft.dy - textBottomLeft.dy, 14.0); // margin (with no bottom padding)
+    expect(actionTextBottomLeft.dx - textBottomRight.dx, 24.0);
+    expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 24.0 + 30.0); // margin + right padding
+    expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 14.0); // margin (with no bottom padding)
+  });
+
   testWidgets('SnackBarClosedReason', (WidgetTester tester) async {
     final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
     bool actionPressed = false;