Changing SnackBar's default vertical padding (#78133)

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

@@ -296,10 +296,29 @@ class SnackBar extends StatefulWidget {
   /// The amount of padding to apply to the snack bar's content and optional
   /// action.
   ///
-  /// If this property is null, then the default depends on the [behavior] and
-  /// the presence of an [action]. The start padding is 24 if [behavior] is
-  /// [SnackBarBehavior.fixed] and 16 if it is [SnackBarBehavior.floating]. If
-  /// there is no [action], the same padding is added to the end.
+  /// If this property is null, the default padding values for:
+  ///
+  /// * [content]
+  ///     * Top and bottom paddings are 14.
+  ///     * Left padding is 24 if [behavior] is [SnackBarBehavior.fixed],
+  ///       16 if [behavior] is [SnackBarBehavior.floating]
+  ///     * Right padding is same as start padding if there is no [action], otherwise 0.
+  /// * [action]
+  ///     * Top and bottom paddings are 14
+  ///     * Left and right paddings are half of [content]'s left padding.
+  ///
+  /// If this property is not null, the padding assignment for:
+  ///
+  /// * [content]
+  ///     * Left, top and bottom paddings are assigned normally.
+  ///     * Right padding is assigned normally if there is no [action], otherwise 0.
+  /// * [action]
+  ///     * Left padding is replaced with half value of right padding.
+  ///     * Top and bottom paddings are assigned normally.
+  ///     * Right padding has an additional half value of right padding.
+  ///       ```dart
+  ///       right + (right / 2)
+  ///       ```
   final EdgeInsetsGeometry? padding;
 
   /// The width of the snack bar.
@@ -523,7 +542,7 @@ class _SnackBarState extends State<SnackBar> {
         children: <Widget>[
           Expanded(
             child: Container(
-              padding: const EdgeInsets.symmetric(vertical: _singleLineVerticalPadding),
+              padding: widget.padding == null ? const EdgeInsets.symmetric(vertical: _singleLineVerticalPadding) : null,
               child: DefaultTextStyle(
                 style: contentTextStyle!,
                 child: widget.content,

packages/flutter/test/material/snack_bar_test.dart

@@ -978,9 +978,8 @@ void main() {
     final Offset snackBarTopRight = tester.getTopRight(materialFinder);
     expect(textBottomLeft.dx - snackBarBottomLeft.dx, padding);
     expect(snackBarTopRight.dx - textTopRight.dx, padding);
-    // The text is given a vertical padding of 14 already.
-    expect(snackBarBottomLeft.dy - textBottomLeft.dy, padding + 14);
-    expect(textTopRight.dy - snackBarTopRight.dy, padding + 14);
+    expect(snackBarBottomLeft.dy - textBottomLeft.dy, padding);
+    expect(textTopRight.dy - snackBarTopRight.dy, padding);
   });
 
   testWidgets('Snackbar width can be customized', (WidgetTester tester) async {