How to change text selection option in flutter?

Output:

You can check the code below:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _controller = new TextEditingController();
  final _textfieldFocusNode = new FocusNode();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Padding(
              padding: EdgeInsets.all(20.0),
              child: GestureDetector(
                // intercept all pointer calls
                behavior: HitTestBehavior.opaque,
                onTap: () {
                  FocusScope.of(context).requestFocus(_textfieldFocusNode);
                },
                onLongPress: () {
                  showMenu(
                    context: context,
                    // TODO: Position dynamically based on cursor or textfield
                    position: RelativeRect.fromLTRB(0.0, 300.0, 300.0, 0.0),
                    items: [
                      PopupMenuItem(
                        child: Row(
                          children: <Widget>[
                            // TODO: Dynamic items / handle click
                            PopupMenuItem(
                              child: Text(
                                "Paste",
                                style: Theme.of(context)
                                    .textTheme
                                    .body2
                                    .copyWith(color: Colors.red),
                              ),
                            ),
                            PopupMenuItem(
                              child: Text("Select All"),
                            ),
                          ],
                        ),
                      ),
                    ],
                  );
                },
                child: IgnorePointer(
                  // ensures textfield doesn't overrule GestureDetector
                  child: TextField(
                    focusNode: _textfieldFocusNode,
                    controller: _controller,
                  ),
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

You can use selectionControls parameter to customize text selection menu:

TextField(selectionControls: MyMaterialTextSelectionControls()),

and implement delegate class like here:

class MyMaterialTextSelectionControls extends MaterialTextSelectionControls {
  // Padding between the toolbar and the anchor.
  static const double _kToolbarContentDistanceBelow = 10.0;
  static const double _kToolbarContentDistance = 8.0;

  /// Builder for material-style copy/paste text selection toolbar.
  @override
  Widget buildToolbar(
    BuildContext context,
    Rect globalEditableRegion,
    double textLineHeight,
    Offset selectionMidpoint,
    List<TextSelectionPoint> endpoints,
    TextSelectionDelegate delegate,
    ClipboardStatusNotifier clipboardStatus,
    Offset? lastSecondaryTapDownPosition,
  ) {
    final TextSelectionPoint startTextSelectionPoint = endpoints[0];
    final TextSelectionPoint endTextSelectionPoint =
        endpoints.length > 1 ? endpoints[1] : endpoints[0];
    final Offset anchorAbove = Offset(
      globalEditableRegion.left + selectionMidpoint.dx,
      globalEditableRegion.top +
          startTextSelectionPoint.point.dy -
          textLineHeight -
          _kToolbarContentDistance,
    );
    final Offset anchorBelow = Offset(
      globalEditableRegion.left + selectionMidpoint.dx,
      globalEditableRegion.top +
          endTextSelectionPoint.point.dy +
          _kToolbarContentDistanceBelow,
    );
    final value = delegate.textEditingValue;
    return MyTextSelectionToolbar(
      anchorAbove: anchorAbove,
      anchorBelow: anchorBelow,
      clipboardStatus: clipboardStatus,
      handleCustomButton: () {
        print(value.selection.textInside(value.text));
        delegate.hideToolbar();
      },
    );
  }
}

class MyTextSelectionToolbar extends StatelessWidget {
  const MyTextSelectionToolbar({
    Key? key,
    required this.anchorAbove,
    required this.anchorBelow,
    required this.clipboardStatus,
    required this.handleCustomButton,
  }) : super(key: key);

  final Offset anchorAbove;
  final Offset anchorBelow;
  final ClipboardStatusNotifier clipboardStatus;
  final VoidCallback? handleCustomButton;

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterialLocalizations(context));

    final List<_TextSelectionToolbarItemData> items =
        <_TextSelectionToolbarItemData>[
      _TextSelectionToolbarItemData(
        onPressed: handleCustomButton ?? () {},
        label: 'Custom button',
      ),
    ];

    int childIndex = 0;
    return TextSelectionToolbar(
      anchorAbove: anchorAbove,
      anchorBelow: anchorBelow,
      toolbarBuilder: (BuildContext context, Widget child) =>
          Container(color: Colors.pink, child: child),
      children: items
          .map((_TextSelectionToolbarItemData itemData) =>
              TextSelectionToolbarTextButton(
                padding: TextSelectionToolbarTextButton.getPadding(
                    childIndex++, items.length),
                onPressed: itemData.onPressed,
                child: Text(itemData.label),
              ))
          .toList(),
    );
  }
}

class _TextSelectionToolbarItemData {
  const _TextSelectionToolbarItemData({
    required this.label,
    required this.onPressed,
  });

  final String label;
  final VoidCallback onPressed;
}

Or checkout this one text_selection_controls

You can use Selectable Widget. click here