How do I clip a variable sized widget in flutter?

Please try the following code.

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: ChatAudioTextBubble("Test message"),
        ),
      ),
    );
  }
}

class ChatAudioTextBubble extends StatefulWidget {
  final String text;

  ChatAudioTextBubble(this.text);

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

class _ChatAudioTextBubbleState extends State<ChatAudioTextBubble>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;

  @override
  void initState() {
    _controller = AnimationController(
      lowerBound: 0.0,
      upperBound: 1.0,
      value: 0.0,
      duration: const Duration(milliseconds: 1000),
      vsync: this,
    )..addListener(() {
        this.setState(() {});
      });
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _onTapHandler() {
    _controller.reset();
    _controller.forward(from: 0.0);
  }

  Widget build(BuildContext context) {
    return GestureDetector(
        behavior: HitTestBehavior.translucent,
        onTap: () {
          _onTapHandler();
          print(
              "Plays the audio and starts the animation which takes _controller.value from 0-1");
        },
        child: Stack(children: [
          Positioned.fill(
              child: Container(
            decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(10),
                color: Color(0xFFE6E6E6)),
          )),
          Container(
              constraints: BoxConstraints(
                  minWidth: 40,
                  maxWidth: MediaQuery.of(context).size.width * 0.70),
              child: _buildTextChild()),
          Positioned.fill(
            child: ClipRRect(
              borderRadius: BorderRadius.only(
                topRight: Radius.circular(10.0),
                bottomRight: Radius.circular(10.0),
              ),
              child: FractionallySizedBox(
                alignment: Alignment.centerLeft,
                heightFactor: 1,
                widthFactor: _controller == null ? 0 : _controller.value,
                child: Container(
                  decoration: BoxDecoration(
                    shape: BoxShape.rectangle,
                    color: Colors.blue.withOpacity(0.3),
                    borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(10.0),
                      topRight: Radius.zero,
                      bottomLeft: Radius.circular(10.0),
                      bottomRight: Radius.zero,
                    ),
                  ),
                ),
              ),
            ),
          ),
        ]));
  }

  Widget _buildTextChild() {
    final color = Colors.black;
    return Padding(
        padding: EdgeInsets.fromLTRB(16, 8, 16, 8),
        child: Text(widget.text, style: TextStyle(color: color, fontSize: 18)));
  }
}