Summary

Allow developers to use a Dialog with StageStyle.EXTENDED.

Problem

Dialogs don't allow developers to install a HeaderBar, which is an obligatory control in order to create dialogs with StageStyle.EXTENDED.

Solution

Add a new DialogPane.headerBar property, which installs a HeaderBar at the top of the dialog window.

Specification

Dialog:

    --- a/modules/javafx.controls/src/main/java/javafx/scene/control/Dialog.java
    +++ b/modules/javafx.controls/src/main/java/javafx/scene/control/Dialog.java
    @@ -461,10 +462,10 @@
         }
     
         /**
    -     * Specifies the style for this dialog. This must be done prior to making
    -     * the dialog visible. The style is one of: StageStyle.DECORATED,
    -     * StageStyle.UNDECORATED, StageStyle.TRANSPARENT, StageStyle.UTILITY,
    -     * or StageStyle.UNIFIED.
    +     * Specifies the style for this dialog. This must be done prior to making the dialog visible.
    +     * <p>
    +     * Note that a dialog with the {@link StageStyle#EXTENDED} style should also specify a {@link HeaderBar} with
    +     * the {@link DialogPane#setHeaderBar(HeaderBar)} method, as otherwise the dialog window will not be draggable.
          *
          * @param style the style for this dialog.
          *

DialogPane:

    --- a/modules/javafx.controls/src/main/java/javafx/scene/control/DialogPane.java
    +++ b/modules/javafx.controls/src/main/java/javafx/scene/control/DialogPane.java
    @@ -430,6 +434,74 @@
             return imageUrl;
         }
     
    +    // --- header bar
    +    private ObjectProperty<HeaderBar> headerBar;
    +
    +    /**
    +     * Specifies the {@link HeaderBar} for the dialog. The {@code HeaderBar} will be placed at the
    +     * top of the dialog window, and extend the entire width of the window. This property will only
    +     * be used if the dialog window is configured with the {@link StageStyle#EXTENDED} style; it has
    +     * no effect for other styles.
    +     *
    +     * @return the {@code headerBar} property
    +     * @defaultValue {@code null}
    +     * @since 26
    +     * @deprecated This is a preview feature which may be changed or removed in a future release.
    +     */
    +    @Deprecated(since = "26")
    +    public final ObjectProperty<HeaderBar> headerBarProperty()
    +
    +    /**
    +     * Gets the value of the {@link #headerBarProperty() headerBar} property.
    +     *
    +     * @return the {@code HeaderBar}
    +     * @since 26
    +     * @deprecated This is a preview feature which may be changed or removed in a future release.
    +     */
    +    @Deprecated(since = "26")
    +    public final HeaderBar getHeaderBar()
    +
    +    /**
    +     * Sets the value of the {@link #headerBarProperty() headerBar} property.
    +     *
    +     * @param value the new value
    +     * @since 26
    +     * @deprecated This is a preview feature which may be changed or removed in a future release.
    +     */
    +    @Deprecated(since = "26")
    +    public final void setHeaderBar(HeaderBar value)