RFR: 8090547: Allow for transparent backgrounds in WebView [v4]

[Kevin Rushforth]

On Fri, 27 Aug 2021 17:34:25 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

>> I'm more worried about correctness than performance. Setting a clip does not necessarily imply that the clipped region should be cleared. So this feels a bit like a workaround for a missing `clearRect` elsewhere. I wonder if there is code somewhere that assumes that if the fill color is fully transparent it can skip the call to `clearRect`?
>
> I guess the assumption might be the other way around? Since the fill color was not transparent, there was no need to clear the area, when rendering the same solid rect at the new position?
> 
> If you check this [comment](https://bugs.openjdk.java.net/browse/JDK-8090547?focusedCommentId=13808421&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13808421), there is already a mention that `WCPageBackBufferImpl::copyArea` doesn't care about the alpha channel.
> 
> The proposed patch: 
> 
>  texture.createGraphics().clearQuad(x+dx, y+dy, x+dx+w, y+dy+h);
> 
>  could work if we could apply it conditionally only for alpha == 0 (or maybe also for alpha < 1).
> 
> My current approach with `clearRect` ultimately calls `clearQuad`, so both might be doing the same after all.

I don't doubt that the area should be cleared. What I was questioning is whether this was the best place to do it? I'd be OK with it if you document it.

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PR: https://git.openjdk.java.net/jfx/pull/563