RFR: 8369836: Update HeaderBar API [v16]

[Michael Strauß]

On Thu, 8 Jan 2026 20:06:54 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> 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.

If you make the mistake of marking an interactive node on the header bar as `DRAGGABLE`, then you still won't be able to DnD this node, as the only events you'll ever receive are `MOUSE_PRESSED` and `MOUSE_RELEASED`, but no intermediate `MOUSE_DRAGGED`. The move-modal loop that moves the window will eat all the intermediate events.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1936#discussion_r2673729440