Repository: flutter
Issue: [web] TextStyle with height < 1 causes ShaderMask not to paint whole text area
Labels: framework,a: typography,platform-web,has reproducible steps,P2,found in release: 2.5,found in release: 2.6,team-web,triaged-web

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## Steps to Reproduce
Setting text widget text style height to less than 1 causes shadermask bounds to be less than text widget size.
Text widget paints correctly. See dartpad for demonstration of problem.

<!-- Please include full steps to reproduce so that we can reproduce the problem. -->
Open dartpad for demonstration.
https://dartpad.dev/?id=361cab5769be71d5cc7e254875ff8544&null_safety=true


**Expected results:** <!-- what did you want to see? -->
ShaderMask paints whole text from top to bottom.

**Actual results:** <!-- what did you see? -->
Shadermask starts the paint from y-offset starting from line height.

<details>
<summary>Code sample</summary>
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      Please create a minimal reproducible sample that shows the problem
      and attach it below between the lines with the backticks.

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      which is capable of creating and running small Flutter apps.

      Without this we will unlikely be able to progress on the issue, and because of that
      we regretfully will have to close it.
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```dart
import 'package:flutter/material.dart';

const 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: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ShaderMask(
      blendMode: BlendMode.srcIn,
              shaderCallback: (Rect bounds) => const LinearGradient(colors: [Colors.white, Colors.green]).createShader(
                      Rect.fromLTWH(0, 0, bounds.width, bounds.height)),
      child: Text(
      'Hello,\nWorld!',
      style: Theme.of(context).textTheme.headline4!.copyWith(height:0.5, color: Colors.red)
      ),
    );
  }
}


```

</details>

<details>
  <summary>Logs</summary>

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```
```

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     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
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```
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
```

</details>

<img width="137" alt="Screenshot 2021-11-20 at 11 05 08" src="https://user-images.githubusercontent.com/442128/142720805-1c860509-f87a-4878-9f89-949e83c410a3.png">

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<img width="137" alt="Screenshot 2021-11-20 at 11 05 08" src="https://user-images.githubusercontent.com/442128/142720805-1c860509-f87a-4878-9f89-949e83c410a3.png">
->