Fix CupertinoTextSelectionToolbar showing unnecessary pagination (#104587)

packages/flutter/lib/src/cupertino/text_selection_toolbar.dart

@@ -103,7 +103,7 @@ class CupertinoTextSelectionToolbar extends StatelessWidget {
   /// default Cupertino toolbar.
   final CupertinoToolbarBuilder toolbarBuilder;
 
-  // Add the visial vertical line spacer between children buttons.
+  // Add the visual vertical line spacer between children buttons.
   static List<Widget> _addChildrenSpacers(List<Widget> children) {
     final List<Widget> nextChildren = <Widget>[];
     for (int i = 0; i < children.length; i++) {
@@ -801,8 +801,9 @@ class _RenderCupertinoTextSelectionToolbarItems extends RenderBox with Container
       double paginationButtonsWidth = 0.0;
       if (currentPage == 0) {
         // If this is the last child, it's ok to fit without a forward button.
+        // Note childCount doesn't include slotted children which come before the list ones.
         paginationButtonsWidth =
-            i == childCount - 1 ? 0.0 : _nextButton!.size.width;
+            i == childCount + 2 ? 0.0 : _nextButton!.size.width;
       } else {
         paginationButtonsWidth = subsequentPageButtonsWidth;
       }

packages/flutter/test/cupertino/text_selection_toolbar_test.dart

@@ -181,6 +181,33 @@ void main() {
     expect(findOverflowBackButton(), findsNothing);
   }, skip: kIsWeb); // [intended] We do not use Flutter-rendered context menu on the Web.
 
+  testWidgets('does not paginate if children fit with zero margin', (WidgetTester tester) async {
+    final List<Widget> children = List<Widget>.generate(7, (int i) => const TestBox());
+    final double spacerWidth = 1.0 / tester.binding.window.devicePixelRatio;
+    final double dividerWidth = 1.0 / tester.binding.window.devicePixelRatio;
+    const double borderRadius = 8.0; // Should match _kToolbarBorderRadius
+    final double width = 7 * TestBox.itemWidth + 6 * (dividerWidth + 2 * spacerWidth) + 2 * borderRadius;
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: SizedBox(
+            width: width,
+            child: CupertinoTextSelectionToolbar(
+              anchorAbove: const Offset(50.0, 100.0),
+              anchorBelow: const Offset(50.0, 200.0),
+              children: children,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // All children fit on the screen, so they are all rendered.
+    expect(find.byType(TestBox), findsNWidgets(children.length));
+    expect(findOverflowNextButton(), findsNothing);
+    expect(findOverflowBackButton(), findsNothing);
+  }, skip: kIsWeb); // [intended] We do not use Flutter-rendered context menu on the Web.
+
   testWidgets('positions itself at anchorAbove if it fits', (WidgetTester tester) async {
     late StateSetter setState;
     const double height = _kToolbarHeight;