RFR: 8369836: Update HeaderBar API [v16]

[Michael Strauß]

On Thu, 8 Jan 2026 19:59:38 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> I've tested this, and you can drop something _onto_ a header bar, but you can't do the oppsite and drag-and-drop the header bar itself. That seems obvious, as any attempt to press-and-drag the header bar will just move the window instead of starting a DnD operation.
>
> I was thinking of a scenario where DnD is used to rearrange items within the header bar, or DnD between the header bar and some palette-like UI, to customize the header bar.

This works just as before. All of the code in `GlassWindow::HandleCaptionMouseEvents` is only ever relevant if you start the PDR gesture by clicking on a draggable area of the header bar. If you instead click on an interactive part (like a rearrangable button bar or something), the `m_caption.tracking` flag will not be set, and the entire method is a NOP.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1936#discussion_r2673719551