RFR: 8090547: Allow for transparent backgrounds in WebView

[Kevin Rushforth]

On Fri, 2 Jul 2021 11:01:56 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

> Currently, `WebPage` has already a public `setBackgroundColor()` method, but the class is not public. Therefore, public API is needed in `WebView` to allow developers access to it.
> 
> In line with the `fontSmoothingType` property, this PR provides public support for setting the background color of a WebPage, by adding a `pageFill` property, and a CSR is required.
> 
> The color for the background, that can be opaque, transparent or with any level of opacity, can be set via code or via CSS using `-fx-page-fill`.
> 
> Unit tests and a system test are provided.

This new API warrants a brief discussion on the mailing list prior to review. Some things to think about:

* Should the new `WebView` property be a `Color` or should it be a `Paint`?

* You propose to add a new CSS element, `-fx-page-fill`. Do we need it or can one of the existing elements work (e.g., `-fx-background-color`)? If we do end up adding a new element, you will need to document it in `cssref.html`.

modules/javafx.web/src/main/java/javafx/scene/web/WebView.java line 706:

> 704:      * Default color: White
> 705:      *
> 706:      * @since JavaFX 17

We no longer use `JavaFX` in the version for `@since` commands. And  it's too late for 17, so this should be:

* @since 18

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