made SelectionArea alignment consistent between web and other platform (flutter#150037)

Currently, when text is placed inside a SelectionArea widget that's nested within a Column widget, it results in misalignment, causing the text to appear centered instead of aligned as intended.

This was originally flutter#149552 but had issues with my branch.

*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*
Fixes flutter#148934 
Fixes flutter#121053 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: LimaneGaya

packages/flutter/lib/src/widgets/_platform_selectable_region_context_menu_web.dart

@@ -135,7 +135,6 @@ class PlatformSelectableRegionContextMenu extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Stack(
-      alignment: Alignment.center,
       children: <Widget>[
         const Positioned.fill(
           child: HtmlElementView(

packages/flutter/test/widgets/selectable_region_test.dart

@@ -3594,6 +3594,27 @@ void main() {
     skip: kIsWeb, // [intended] Web uses its native context menu.
   );
 
+  // Regression test for https://github.com/flutter/flutter/issues/121053.
+  testWidgets('Ensure SelectionArea does not affect the layout of its children', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: <Widget>[
+              SelectionArea(child: Text('row 1')),
+              Text('row 2'),
+            ],
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    final double xOffset1 = tester.getTopLeft(find.text('row 1')).dx;
+    final double xOffset2 = tester.getTopLeft(find.text('row 2')).dx;
+    expect(xOffset1, xOffset2);
+  },
+    variant: TargetPlatformVariant.all(),
+  );
+
   testWidgets('the selection behavior when clicking `Copy` item in mobile platforms', (WidgetTester tester) async {
     List<ContextMenuButtonItem> buttonItems = <ContextMenuButtonItem>[];
     final FocusNode focusNode = FocusNode();