8343956: Focus delegation API

[mstr2]

Implementation of focus delegation.

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Progress

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Issue

• JDK-8343956: Focus delegation API (Enhancement - P4)

Reviewing

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Update a local copy of the PR:
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https://git.openjdk.org/jfx/pull/1632.diff

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[beldenfox]

PopupWindows have to solve a similar problem; multiple nodes can receive key events and they all act as if they have focus. This gets (mildly) complicated when dealing with input methods. There may be multiple focused nodes but there can only be one caret so you have to have a mechanism to determine which node will respond to input method requests. You also have to determine when to enable and disable IM events at the OS level. Spinners have never dealt with input methods but ComboBoxes do.

I've submitted PR #1634 to get input methods working in popups. If you want input methods to work with focus delegation you would need to do similar bookkeeping.

I noticed that PopupWindows use EventRedirectors for forwarding messages. I didn't dig into the EventRedirector implementation but it is an existing class that seems to be tackling the same problem as this PR.