RFR: 8370446: Support dialogs with StageStyle.EXTENDED (Preview) [v5]

[Kevin Rushforth]

On Sat, 15 Nov 2025 02:18:49 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> Adds the `DialogPane.headerBar` property, which allows developers to specify a custom `HeaderBar` when the dialog uses the `EXTENDED` stage style.
>
> Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains nine commits:
> 
>  - Merge branch 'master' into feature/extended-dialog
>    
>    # Conflicts:
>    #	modules/javafx.controls/src/test/java/test/javafx/scene/control/DialogPaneTest.java
>  - fix incorrect button metrics
>  - documentation
>  - Merge branch 'master' into feature/extended-dialog
>  - move HeaderBar to DialogPane
>  - document/verify preview feature
>  - Ensure that DialogPane is always attached to Scene
>  - Lazily initialize Dialog.headerBar
>  - Support dialogs with StageStyle.EXTENDED

The API and docs looks good.

The code changes look reasonable, but I haven't tested them and also want to take another pass, since these have the potential to impact DialogPane even when not using `StageStyle.EXTENDED` + `HeaderBar`.

-------------

PR Review: https://git.openjdk.org/jfx/pull/1943#pullrequestreview-3474360777