RFR: 8349096: Split/MenuButton: exception initializing in a background thread

[Kevin Rushforth]

On Wed, 12 Feb 2025 20:15:03 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

> ## Root Cause
> 
> The ContextMenu (PopupWindow) cannot be shown in a background thread.
> 
> ## Solution
> 
> Bail out of `show()` if in a background thread.

One more reason to disallow show in this case is that ContextMenu is a descendent of PopupWindow, and whatever else we do, we need to disallow calling `PopupWindow::show` from a background thread.

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PR Comment: https://git.openjdk.org/jfx/pull/1709#issuecomment-2655097696