Description

This PR change Slider can not be painted in a narrower constraint like other Material Widgets such as Switch.

If I just remove the assert checks, the fellow demo code paint Slider incorrectly below(the active track and inactive track switched mistake), so I modify the getPreferredRect logic.

After change:

The issue #64629 root cause is that, when minimizing the window the screen size will change to zero and trigger a flushPaint.
This change does make the appearance of the issue disappear, but I am not sure whether the window size change to zero is intended, I think we can track that at a new issue, right?

DEMO

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

void main() {
  debugPaintLayerBordersEnabled = true;
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double value = 0.5;
  _onChange(newValue) {
    setState(() {
      print('$newValue');
      value = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Directionality(
      textDirection: TextDirection.ltr,
      child: Center(
        child: RepaintBoundary(
          child: SizedBox(
            width: 10,
            height: 10,
            child: Slider(
              value: value,
              onChanged: _onChange,
            ),
          ),
        ),
      ),
    ));
  }
}

Related Issues

Fixes #64629

Tests

I added the following tests:

See files.

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