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

[Jose Pereda]

On Fri, 27 Aug 2021 13:05:24 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> Initially, this was needed when there was some level of transparency: when scrolling the old content was not cleared and you could see it at its old position.
>> 
>> For the full transparency case, this is still the case, but for translucent colors (0 < opacity < 1) I can't reproduce it anymore, so I'll modify this to apply only `if (isTransparent) { clearRect(); }`.
>> 
>> I don't see a performance drop because of this.
>
> 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.

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