RFR: 8370446: Support dialogs with StageStyle.EXTENDED

Thu Oct 23 16:27:23 UTC 2025

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.

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Commit messages:
 - document/verify preview feature
 - Ensure that DialogPane is always attached to Scene
 - Lazily initialize Dialog.headerBar
 - Support dialogs with StageStyle.EXTENDED

Changes: https://git.openjdk.org/jfx/pull/1943/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370446
  Stats: 158 lines in 4 files changed: 135 ins; 13 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1943.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1943/head:pull/1943

PR: https://git.openjdk.org/jfx/pull/1943