RFR: 8370446: Support dialogs with StageStyle.EXTENDED

[Kevin Rushforth]

On Tue, 21 Oct 2025 23:54:55 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> Adds the `Dialog.headerBar` property, which allows developers to specify a custom `HeaderBar` when the dialog uses the `EXTENDED` stage style. The property is ignored for all other stage styles.

modules/javafx.controls/src/main/java/javafx/scene/control/Dialog.java line 594:

> 592:      * @since 26
> 593:      */
> 594:     private ObjectProperty<HeaderBar> headerBar;

Since this is a preview feature, it needs to be documented and handled as such.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1943#discussion_r2455576341