[CP] Use PorterDuff blend mode when below Android 29

[bparrishMines]

issue_link

#104686

Commit Hash

092481d

Target

beta

pr_link

#108135

Impacted Users

Customers and plugins using Android Platform Views

Impact Description

On Android versions below 29, platform views struggle to render when the background is transparent:

Workaround

Any plugin or app that uses platform views could switch to Hybrid Composition when the background is made transparent.

Risk

low

Test Coverage

yes

Validation Steps

1. Build a custom engine that changes:

https://github.com/flutter/engine/blob/main/shell/platform/android/io/flutter/plugin/platform/PlatformViewWrapper.java#L172

canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);

to

canvas.drawColor(Color.TRANSPARENT);

2. Create a Flutter app with webview_flutter. The example app from the plugin also works: webview_flutter example app

3. Set the background to transparent: https://github.com/flutter/plugins/blob/main/packages/webview_flutter/webview_flutter_android/example/lib/main.dart#L137

4. Create and start an Android emulator as Nexus 4 API 28.

5. Run the app and navigate through any webpage.

6. Compare the result to adding the PorterDuff.Mode.CLEAR parameter in step 1.

[bparrishMines]

cc @stuartmorgan

[itsjustkevin]

Thanks @bparrishMines! As soon as approval from @stuartmorgan comes in, I will push this to the release queue for the next beta hotfix.

[stuartmorgan]

I approve flutter/engine#34800 for cherry picking to the beta branch!

I'm assuming we do that by cherry-picking that PR's engine commit and then rolling the beta branch's engine to that new engine commit though. The links above are to the master engine roll, which would give us not the specific engine PR, but everything that's landed in the engine's main since the branch point.

[itsjustkevin]

Correct @stuartmorgan, I think the commit @bparrishMines was looking for is 7025645 at flutter/engine#34800.

Is this the correct commit you'd like to cherry-pick into the next beta hotfix?

[bparrishMines]

@itsjustkevin Yes, that's the commit. I created: #108338. Let me know if this is what you were looking for.

[itsjustkevin]

Thanks @bparrishMines, monitoring the cherry-pick.

[godofredoc]

CP has landed in the release candidate branch.

[github-actions]

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