Repository: flutter
Issue: Flutter container bottom-center aligned in a stack not fully aligned at the bottom
Labels: framework,f: material design,a: fidelity,platform-windows,platform-web,e: web_canvaskit,has reproducible steps,P2,found in release: 2.3,team-web,triaged-web

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run `flutter create bug`.
2. Update the files as follows: 
Replace main.dart with:
```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Container(
            decoration: BoxDecoration(
              color: Colors.black,
            ),
            child: Center(
              child: Container(
                  height: 300,
                  width: 300,
                  child: Stack(children: [
                    Container(color: Colors.yellow),
                    Align(
                        alignment: Alignment.bottomCenter,
                        child: SizedBox(
                            height: 50,
                            child: Container(
                              color: Colors.grey.shade900.withOpacity(0.9),
                            )))
                  ])),
            )));
  }
}

```
3. Run the application in Chrome

**Expected results:**
The bottom centered container should show no yellow on the bottom edge, the left edge or the right edge.  Sometimes it does as you resize Chrome's window size.

**Actual results:**
The bottom centered container sometimes shows yellow on the bottom edge, the left edge or the right edge as you resize Chrome's window size.  

As shown here: https://i.stack.imgur.com/CF6Ol.png

Please see this more more details: https://stackoverflow.com/questions/67539672/flutter-container-bottom-center-aligned-in-a-stack-not-fully-aligned-at-the-bott/67539863?noredirect=1#comment119380349_67539863

<details>
  <summary>Logs</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
      exception, please see if the error message includes enough information
      to explain how to solve the issue.
-->

```
PS C:\NonSys\FlutterBug\myapp> flutter run --verbose
[  +84 ms] executing: [C:\NonSys\flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[  +94 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[   +1 ms] 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[   +1 ms] executing: [C:\NonSys\flutter/] git tag --points-at 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[  +72 ms] Exit code 0 from: git tag --points-at 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[        ] 2.0.6
[  +38 ms] executing: [C:\NonSys\flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[  +48 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] origin/stable
[        ] executing: [C:\NonSys\flutter/] git ls-remote --get-url origin
[  +45 ms] Exit code 0 from: git ls-remote --get-url origin
[        ] https://github.com/flutter/flutter.git
[  +83 ms] executing: [C:\NonSys\flutter/] git rev-parse --abbrev-ref HEAD
[  +47 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] stable
[  +79 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[   +3 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +61 ms] executing: C:\Users\terence.darwen\AppData\Local\Android\sdk\platform-tools\adb.exe devices -l
[  +49 ms] List of devices attached
[   +5 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[   +4 ms] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +73 ms] Multiple devices found:
[  +23 ms] Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.19042.985]
[   +1 ms] Chrome (web)      • chrome  • web-javascript • Google Chrome 90.0.4430.212
[   +2 ms] [1]: Windows (windows)
[        ] [2]: Chrome (chrome)
[   +2 ms] Please choose one (To quit, press "q/Q")
[        ] : 
[+13862 ms] Q
[  +11 ms] "flutter run" took 14,124ms.
[  +17 ms] 
           #0      throwToolExit (package:flutter_tools/src/base/common.dart:10:3)
           #1      DeviceManager._chooseOneOfAvailableDevices (package:flutter_tools/src/device.dart:306:7)
           <asynchronous suspension>
           #2      DeviceManager.findTargetDevices (package:flutter_tools/src/device.dart:294:37)
           <asynchronous suspension>
           #3      FlutterCommand.findAllTargetDevices (package:flutter_tools/src/runner/flutter_command.dart:1193:28)
           <asynchronous suspension>
           #4      RunCommand.validateCommand (package:flutter_tools/src/commands/run.dart:440:15)
           <asynchronous suspension>
           #5      FlutterCommand.verifyThenRunCommand (package:flutter_tools/src/runner/flutter_command.dart:1114:5)
           <asynchronous suspension>
           #6      FlutterCommand.run.<anonymous closure> (package:flutter_tools/src/runner/flutter_command.dart:1009:27)
           <asynchronous suspension>
           #7      AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:150:19)
           <asynchronous suspension>
           #8      AppContext.run (package:flutter_tools/src/base/context.dart:149:12)
           <asynchronous suspension>
           #9      CommandRunner.runCommand (package:args/command_runner.dart:197:13)
           <asynchronous suspension>
           #10     FlutterCommandRunner.runCommand.<anonymous closure> (package:flutter_tools/src/runner/flutter_command_runner.dart:278:9)
           <asynchronous suspension>
           #11     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:150:19)
           <asynchronous suspension>
           #12     AppContext.run (package:flutter_tools/src/base/context.dart:149:12)
           #13     FlutterCommandRunner.runCommand (package:flutter_tools/src/runner/flutter_command_runner.dart:234:5)
           <asynchronous suspension>
           #14     run.<anonymous closure>.<anonymous closure> (package:flutter_tools/runner.dart:64:9)
           <asynchronous suspension>
           #15     run.<anonymous closure> (package:flutter_tools/runner.dart:62:12)
           <asynchronous suspension>
           #16     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:150:19)
           <asynchronous suspension>
           #17     AppContext.run (package:flutter_tools/src/base/context.dart:149:12)
           <asynchronous suspension>
           #18     runInContext (package:flutter_tools/src/context_runner.dart:73:10)
           <asynchronous suspension>
           #19     main (package:flutter_tools/executable.dart:90:3)
           <asynchronous suspension>


[ +161 ms] ensureAnalyticsSent: 150ms
[   +2 ms] Running shutdown hooks
[        ] Shutdown hooks complete
[   +1 ms] exiting with code 1
PS C:\NonSys\FlutterBug\myapp> flutter run --verbose
[  +89 ms] executing: [C:\NonSys\flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[  +87 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[        ] 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[        ] executing: [C:\NonSys\flutter/] git tag --points-at 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[  +73 ms] Exit code 0 from: git tag --points-at 1d9032c7e1d867f071f2277eb1673e8f9b0274e3
[        ] 2.0.6
[  +38 ms] executing: [C:\NonSys\flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[  +49 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] origin/stable
[        ] executing: [C:\NonSys\flutter/] git ls-remote --get-url origin
[  +46 ms] Exit code 0 from: git ls-remote --get-url origin
[        ] https://github.com/flutter/flutter.git
[  +87 ms] executing: [C:\NonSys\flutter/] git rev-parse --abbrev-ref HEAD
[  +46 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] stable
[  +73 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[   +2 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +60 ms] executing: C:\Users\terence.darwen\AppData\Local\Android\sdk\platform-tools\adb.exe devices -l
[  +48 ms] List of devices attached
[   +5 ms] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[   +4 ms] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +73 ms] Multiple devices found:
[  +25 ms] Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.19042.985]
[   +1 ms] Chrome (web)      • chrome  • web-javascript • Google Chrome 90.0.4430.212
[   +1 ms] [1]: Windows (windows)
[        ] [2]: Chrome (chrome)
[   +2 ms] Please choose one (To quit, press "q/Q")
[        ] : 
[+8685 ms] 2
[  +12 ms] Skipping pub get: version match.
[ +121 ms] Generating C:\NonSys\FlutterBug\myapp\android\app\src\main\java\io\flutter\plugins\GeneratedPluginRegistrant.java
[  +99 ms] Launching lib\main.dart on Chrome in debug mode...
[ +123 ms] Updating assets
[  +76 ms] Waiting for connection from debug service on Chrome...
[  +93 ms] <- reset
[   +9 ms] C:\NonSys\flutter\bin\cache\dart-sdk\bin\dart.exe --disable-dart-dev C:\NonSys\flutter\bin\cache\artifacts\engine\windows-x64\frontend_server.dart.snapshot --sdk-root C:\NonSys\flutter\bin\cache\flutter_web_sdk/
--incremental --target=dartdevc --debugger-module-names --experimental-emit-debug-metadata -DFLUTTER_WEB_AUTO_DETECT=true --output-dill C:\Users\TERENC~1.DAR\AppData\Local\Temp\flutter_tools.1aae4075\flutter_tool.aafa09d\app.dill--libraries-spec file:///C:/NonSys/flutter/bin/cache/flutter_web_sdk/libraries.json --packages C:\NonSys\FlutterBug\myapp\.dart_tool\package_config.json -Ddart.vm.profile=false -Ddart.vm.product=false --enable-asserts
--track-widget-creation --filesystem-root C:\Users\TERENC~1.DAR\AppData\Local\Temp\flutter_tools.1aae4075\flutter_tools.f9c8dcf1 --filesystem-scheme org-dartlang-app --initialize-from-dill
build\c7922d95bf4a2462b75c84a97c312edb.cache.dill.track.dill --platform file:///C:/NonSys/flutter/bin/cache/flutter_web_sdk/kernel/flutter_ddc_sdk.dill --no-sound-null-safety
[  +12 ms] <- compile org-dartlang-app:/web_entrypoint.dart
[+15745 ms] Waiting for connection from debug service on Chrome... (completed in 15.8s)
[   +1 ms] Synced 29.1MB.
[        ] <- accept
[        ] Caching compiled dill
[ +247 ms] Using Opening in existing browser session.

[ +905 ms] [CHROME]:
[  +11 ms] [CHROME]:DevTools listening on ws://127.0.0.1:59992/devtools/browser/b4879423-f97a-43e3-8400-2907a787d1ee
[ +217 ms] DwdsInjector: Received request for entrypoint at http://localhost:59970/main_module.bootstrap.js
[   +6 ms] MetadataProvider: Loading debug metadata...
[  +12 ms] MetadataProvider: Loaded debug metadata
[   +6 ms] DwdsInjector: Injected debugging metadata for entrypoint at http://localhost:59970/main_module.bootstrap.js
[+5282 ms] DevHandler: Debug service listening on ws://127.0.0.1:60035/EI36EbYj_Y8=/ws

[   +9 ms] Debug service listening on ws://127.0.0.1:60035/EI36EbYj_Y8=/ws
[        ] Running with unsound null safety
[        ] For more information see https://dart.dev/null-safety/unsound-null-safety
[   +2 ms]   To hot restart changes while running, press "r" or "R".
[        ] For a more detailed help message, press "h". To quit, press "q".
[+42618 ms] DevHandler: Stopped debug service on ws://127.0.0.1:60033

[+4165 ms] Application finished.
[   +4 ms] "flutter run" took 78,747ms.
[  +82 ms] ensureAnalyticsSent: 79ms
[   +1 ms] Running shutdown hooks
[        ] Shutdown hook priority 4
[ +173 ms] Shutdown hooks complete
[   +1 ms] exiting with code 0
```

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
```

</details>