Set GLArea.has-alpha = true

[darkwater]

GLArea.has-alpha defaults to false, which makes it impossible for a Flutter app to be transparent.

The additional change to glClearColor is to prevent a black flash when the application starts, before Flutter widgets become visible. This will also affect applications that aren't transparent, by showing the default GTK background color instead of black. This will not make the window transparent, because the developer currently has to add some code to their linux/my_application.cc to support transparency.

Fixes flutter/flutter#152154

Transparent windows were possible before #50754. Would it be ok to add a test for this to prevent breaking transparency again in the future?

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[robert-ancell]

Great! I hadn't noticed that.

[cbracken]

Oh nice! We were just discussing this issue other day! Thanks for the fix. lgtm!

[Hixie]

This will also affect applications that aren't transparent, by showing the default GTK background color instead of black.

This is a regression -- the default behaviour of a Flutter app that paints nothing is a black window. This breaks applications that depend on the default background being black.

[Hixie]

Can we revert this? It landed without tests and with no test exemption and regressed behaviour.

[cbracken]

Reason for revert: Regression + no tests. Must have been at the end of a long day, cause I missed the missing test and glaring warning about that.

@darkwater see the MacOS implementation (On FlutterViewController) and the tests in shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm for how you might approach testing this. It's fine to offer the ability to set the background colour of the view via API exposed to the Runner, but the default should remain black.

[auto-submit]

Time to revert pull request flutter/engine/54053 has elapsed.
You need to open the revert manually and process as a regular pull request.

[cbracken]

Revert out in #54190

[robert-ancell]

This was fixes as part of #54575