Overlay is not displayed properly on web

[melwinlobo18]

To Reproduce
Steps to reproduce the behavior:

1. Run the example project in flutter web

Expected behavior
The background is not greyed out while displaying the showcase.

Screenshots

Desktop (please complete the following information):

• OS: Windows
• Browser: Chrome
• Version: 85.0.4183.121

[√] Flutter (Channel beta, 1.22.1, on Microsoft Windows [Version 10.0.18362.1082], locale en-IN)
    • Flutter version 1.22.1 at C:\src\flutter
    • Framework revision f30b7f4db9 (7 days ago), 2020-10-08 10:06:30 -0700
    • Engine revision 75bef9f6c8
    • Dart version 2.10.1


[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at C:\Users\I524760\AppData\Local\Android\sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 4.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 48.1.2
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] VS Code (version 1.50.0)
    • VS Code at C:\Users\I524760\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.15.0

[√] Connected device (3 available)
    • Web Server (web) • web-server • web-javascript • Flutter Tools
    • Chrome (web)     • chrome     • web-javascript • Google Chrome 85.0.4183.121
    • Edge (web)       • edge       • web-javascript • Microsoft Edge 86.0.622.38

• No issues found!

[sanket-simform]

@melwinlobo18
We're using custom paint to build overlay and custom paint not working properly with CanvasKit so it requires SKIA.
So currently solution is to use below config and it will work as expected.

flutter build web --release --dart-define=FLUTTER_WEB_USE_SKIA=true

Note: We haven't tested plugin much with web yet.