Add missing margin to SnackBarAction (#67167)

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

@@ -427,6 +427,8 @@ class _SnackBarState extends State<SnackBar> {
     final EdgeInsetsGeometry padding = widget.padding
       ?? EdgeInsetsDirectional.only(start: horizontalPadding, end: widget.action != null ? 0 : horizontalPadding);
 
+    final double actionHorizontalMargin = (widget.padding?.resolve(TextDirection.ltr).right ?? horizontalPadding) / 2;
+
     final CurvedAnimation heightAnimation = CurvedAnimation(parent: widget.animation!, curve: _snackBarHeightCurve);
     final CurvedAnimation fadeInAnimation = CurvedAnimation(parent: widget.animation!, curve: _snackBarFadeInCurve);
     final CurvedAnimation fadeOutAnimation = CurvedAnimation(
@@ -450,14 +452,17 @@ class _SnackBarState extends State<SnackBar> {
             ),
           ),
           if (widget.action != null)
-            TextButtonTheme(
-              data: TextButtonThemeData(
-                style: TextButton.styleFrom(
-                  primary: buttonColor,
-                  padding: EdgeInsets.symmetric(horizontal: horizontalPadding),
+            Padding(
+              padding: EdgeInsets.symmetric(horizontal: actionHorizontalMargin),
+              child: TextButtonTheme(
+                data: TextButtonThemeData(
+                  style: TextButton.styleFrom(
+                    primary: buttonColor,
+                    padding: EdgeInsets.symmetric(horizontal: horizontalPadding),
+                  ),
                 ),
+                child: widget.action!,
               ),
-              child: widget.action!,
             ),
         ],
       ),

packages/flutter/test/material/snack_bar_test.dart

@@ -972,8 +972,8 @@ void main() {
 
     expect(textBottomLeft.dx - snackBarBottomLeft.dx, 24.0 + 10.0); // margin + left padding
     expect(snackBarBottomLeft.dy - textBottomLeft.dy, 17.0 + 40.0); // margin + bottom padding
-    expect(actionTextBottomLeft.dx - textBottomRight.dx, 24.0);
-    expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 24.0 + 30.0); // margin + right padding
+    expect(actionTextBottomLeft.dx - textBottomRight.dx, 24.0 + 12.0); // action padding + margin
+    expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 24.0 + 12.0 + 30.0); // action (padding + margin) + right padding
     expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 17.0 + 40.0); // margin + bottom padding
   });
 
@@ -1027,8 +1027,8 @@ void main() {
 
       expect(textBottomLeft.dx - snackBarBottomLeft.dx, 24.0 + 10.0); // margin + left padding
       expect(snackBarBottomLeft.dy - textBottomLeft.dy, 17.0); // 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(actionTextBottomLeft.dx - textBottomRight.dx, 24.0 + 12.0); // action padding + margin
+      expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 24.0 + 12.0 + 30.0); // action (padding + margin) + right padding
       expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 17.0); // margin (with no bottom padding)
     });
 
@@ -1132,8 +1132,8 @@ void main() {
 
     expect(textBottomLeft.dx - snackBarBottomLeft.dx, 31.0 + 10.0); // margin + left padding
     expect(snackBarBottomLeft.dy - textBottomLeft.dy, 27.0); // margin (with no bottom padding)
-    expect(actionTextBottomLeft.dx - textBottomRight.dx, 16.0);
-    expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 31.0 + 30.0); // margin + right padding
+    expect(actionTextBottomLeft.dx - textBottomRight.dx, 16.0 + 8.0); // action padding + margin
+    expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 31.0 + 30.0 + 8.0); // margin + right (padding + margin)
     expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 27.0); // margin (with no bottom padding)
   });
 
@@ -1190,8 +1190,8 @@ void main() {
 
       expect(textBottomLeft.dx - snackBarBottomLeft.dx, 31.0 + 10.0); // margin + left padding
       expect(snackBarBottomLeft.dy - textBottomLeft.dy, 27.0); // margin (with no bottom padding)
-      expect(actionTextBottomLeft.dx - textBottomRight.dx, 16.0);
-      expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 31.0 + 30.0); // margin + right padding
+      expect(actionTextBottomLeft.dx - textBottomRight.dx, 16.0 + 8.0); // action (margin + padding)
+      expect(snackBarBottomRight.dx - actionTextBottomRight.dx, 31.0 + 30.0 + 8.0); // margin + right (padding + margin)
       expect(snackBarBottomRight.dy - actionTextBottomRight.dy, 27.0); // margin (with no bottom padding)
     });
 
@@ -1942,6 +1942,79 @@ void main() {
         expect(snackBarBottomRight.dy, equals(fabTopRight.dy));
       },
     );
+
+    testWidgets(
+      'SnackBar has correct end padding when it contains an action with fixed behavior',
+      (WidgetTester tester) async {
+        await tester.pumpWidget(
+          MaterialApp(
+            home: Scaffold(
+              body: Builder(
+                builder: (BuildContext context) {
+                    return GestureDetector(
+                      onTap: () {
+                        Scaffold.of(context)!.showSnackBar(SnackBar(
+                          content: const Text('Some content'),
+                          behavior: SnackBarBehavior.fixed,
+                          action: SnackBarAction(
+                            label: 'ACTION',
+                            onPressed: () {},
+                          ),
+                        ));
+                      },
+                      child: const Text('X'),
+                    );
+                },
+              ),
+            ),
+          ),
+        );
+
+        await tester.tap(find.text('X'));
+        await tester.pumpAndSettle();
+
+        final Offset snackBarTopRight = tester.getTopRight(find.byType(SnackBar));
+        final Offset actionTopRight = tester.getTopRight(find.byType(SnackBarAction));
+
+        expect(snackBarTopRight.dx - actionTopRight.dx, 12.0);
+      },
+    );
+
+    testWidgets(
+      'SnackBar has correct end padding when it contains an action with floating behavior',
+      (WidgetTester tester) async {
+        await tester.pumpWidget(
+          MaterialApp(
+            home: Scaffold(
+              body: Builder(
+                builder: (BuildContext context) {
+                    return GestureDetector(
+                      onTap: () {
+                        Scaffold.of(context)!.showSnackBar(SnackBar(
+                          content: const Text('Some content'),
+                          behavior: SnackBarBehavior.floating,
+                          action: SnackBarAction(
+                            label: 'ACTION',
+                            onPressed: () {},
+                          ),
+                        ));
+                      },
+                      child: const Text('X'),
+                    );
+                },
+              ),
+            ),
+          ),
+        );
+
+        await tester.tap(find.text('X'));
+        await tester.pumpAndSettle();
+        final Offset snackBarTopRight = tester.getTopRight(find.byType(SnackBar));
+        final Offset actionTopRight = tester.getTopRight(find.byType(SnackBarAction));
+
+        expect(snackBarTopRight.dx - actionTopRight.dx, 8.0 + 15.0); // button margin + horizontal scaffold outside margin
+      },
+    );
   });
 
   testWidgets('SnackBars hero across transitions when using ScaffoldMessenger', (WidgetTester tester) async {