[Bug]: <webview> transparency broken

[miniak]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Electron Version

14.2.0 / 15.3.0 / 16.0.0-beta.6

What operating system are you using?

macOS

Operating System Version

macOS Catalina 10.15.7

What arch are you using?

x64

Last Known Working Electron version

14.0.0 / 15.0.0-beta.2

Expected Behavior

<webview> element should support trasparency. Check the included Fiddle sample.

Actual Behavior

<webview> element transparency is broken

Testcase Gist URL

No response

Additional Information

webview-transparency.zip

Related to:

electron/shell/browser/api/electron_api_web_contents.cc

Line 1461 in 2a2a1a8

rwhv->SetBackgroundColor(color.value_or(SK_ColorWHITE));

It regressed in this ranges:

• v14.0.0...v14.0.1 (probably fix: BrowserWindow backgroundColor #30819)
• v15.0.0-beta.2...v15.0.0-beta.3 (probably fix: BrowserWindow backgroundColor #30777)

[zcbenz]

/cc @samuelmaddock @codebytere as this is related to #30136 and #30777.

[miniak]

The code was also changed by #31003

[zcbenz]

Can you check if #31685 can fix this?

Also @VerteDinde.

[miniak]

@zcbenz it does not fix this particular issue

[miniak]

This issue is quite a big blocker for us due to how we use <webview> to bridge our legacy AngularJS based part of the app with new ReactJS based UI

[miniak]

@codebytere #31685 is not fixing this bug. I've tested my Fiddle repro with the PR branch locally.
I played with the code a bit and it looks like we need to explicitly call rwhv->SetBackgroundColor(SK_ColorTRANSPARENT) for the webview.