flutter自带的appbar框架无法自定义下划线的宽度和圆角，需要自定义，如下

MyUnderlineTabIndicator

MyUnderlineTabIndicator 为UnderlineTabIndicator 的拷贝，修改了两处源码实现了下标的宽度固定和圆角功能

// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

/// Used with [TabBar.indicator] to draw a horizontal line below the
/// selected tab.
///
/// The selected tab underline is inset from the tab's boundary by [insets].
/// The [borderSide] defines the line's color and weight.
///
/// The [TabBar.indicatorSize] property can be used to define the indicator's
/// bounds in terms of its (centered) widget with [TabIndicatorSize.label],
/// or the entire tab with [TabIndicatorSize.tab].
class MyUnderlineTabIndicator extends Decoration {
  /// Create an underline style selected tab indicator.
  ///
  /// The [borderSide] and [insets] arguments must not be null.
  const MyUnderlineTabIndicator({
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
  })  : assert(borderSide != null),
        assert(insets != null);

  /// The color and weight of the horizontal line drawn below the selected tab.
  final BorderSide borderSide;

  /// Locates the selected tab's underline relative to the tab's boundary.
  ///
  /// The [TabBar.indicatorSize] property can be used to define the
  /// tab indicator's bounds in terms of its (centered) tab widget with
  /// [TabIndicatorSize.label], or the entire tab with [TabIndicatorSize.tab].
  final EdgeInsetsGeometry insets;

  @override
  Decoration lerpFrom(Decoration a, double t) {
    if (a is UnderlineTabIndicator) {
      return UnderlineTabIndicator(
        borderSide: BorderSide.lerp(a.borderSide, borderSide, t),
        insets: EdgeInsetsGeometry.lerp(a.insets, insets, t),
      );
    }
    return super.lerpFrom(a, t);
  }

  @override
  Decoration lerpTo(Decoration b, double t) {
    if (b is MyUnderlineTabIndicator) {
      return MyUnderlineTabIndicator(
        borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
        insets: EdgeInsetsGeometry.lerp(insets, b.insets, t),
      );
    }
    return super.lerpTo(b, t);
  }

  @override
  _MyUnderlinePainter createBoxPainter([VoidCallback onChanged]) {
    return _MyUnderlinePainter(this, onChanged);
  }
}

class _MyUnderlinePainter extends BoxPainter {
  _MyUnderlinePainter(this.decoration, VoidCallback onChanged)
      : assert(decoration != null),
        super(onChanged);

  final MyUnderlineTabIndicator decoration;

  BorderSide get borderSide => decoration.borderSide;
  EdgeInsetsGeometry get insets => decoration.insets;

  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
    assert(rect != null);
    assert(textDirection != null);
    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);

    //希望的宽度
    double wantWidth = 32;
    //取中间坐标
    double cw = (indicator.left + indicator.right) / 2;
    return Rect.fromLTWH(cw - wantWidth / 2,
        indicator.bottom - borderSide.width, wantWidth, borderSide.width);
  }

  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
    assert(configuration != null);
    assert(configuration.size != null);
    final Rect rect = offset & configuration.size;
    final TextDirection textDirection = configuration.textDirection;
    final Rect indicator =
        _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
    // 改为圆角
    final Paint paint = borderSide.toPaint()..strokeCap = StrokeCap.round;
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}

 使用案例：

import 'package:flutter/cupertino.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:trade_app/base/baseColor.dart';
import 'package:trade_app/base/baseSize.dart';
import 'package:trade_app/pages/utils/icon_utils.dart';
import 'package:trade_app/widgets/extend/my_underline_indicator.dart';

///参考 https://www.imooc.com/article/300411
class MineStencilPage extends StatefulWidget {
  @override
  MineStencilPageState createState() => MineStencilPageState();
}

//使用with SingleTickerProviderStateMixin 防止页面被回收
class MineStencilPageState extends State<MineStencilPage>
    with SingleTickerProviderStateMixin {
  TabController _tabController;

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

  @override
  void initState() {
    super.initState();
    _tabController = new TabController(vsync: this, length: 2);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        elevation: 0,
        brightness: Brightness.light,
        leading: IconButton(
            icon: Image.asset(IconUtils.getIconPath('fanhui')),
            onPressed: () => Navigator.pop(context)),
        title: Text(
          '打印配置',
          style: TextStyle(color: BaseColor.colorFF262626, fontSize: 18),
        ),
        backgroundColor: Colors.white,
        bottom: _tabBarTop(),
      ),
      body: Container(child: _tabBarView()),
      backgroundColor: BaseColor.colorFFF5F5F5,
    );
  }

  _tabBarView() => TabBarView(
          controller: _tabController,
          physics: NeverScrollableScrollPhysics(),
          children: <Widget>[
            Center(child: Text('烟草默认模版')),
            Center(child: Text('商超默认模版')),
          ]);

  _tabBarTop() => TabBar(
        tabs: <Widget>[
          Tab(text: '烟草默认模版'),
          Tab(text: '商超默认模版'),
        ],
        controller: _tabController,
        indicatorColor: BaseColor.colorFF03B798,
        indicatorWeight: 3,
        indicatorSize: TabBarIndicatorSize.label,
        indicator: MyUnderlineTabIndicator(
            borderSide: BorderSide(width: 3.0, color: BaseColor.colorFF03B798)),
        indicatorPadding: EdgeInsets.all(5),
        labelColor: BaseColor.colorFF262626,
        labelStyle: TextStyle(fontSize: BaseSize.sp(14)),
        unselectedLabelColor: BaseColor.colorFF999999,
        unselectedLabelStyle: TextStyle(fontSize: BaseSize.sp(14)),
      );
}