RFR: 8351357: Add canary system test checking if Stage receives focus on show [v4]

[Lukasz Kostyra]

On Wed, 2 Jul 2025 06:47:22 GMT, Lukasz Kostyra <lkostyra at openjdk.org> wrote:

>> I thought about that too - but why wouldn't the client area, the scene itself, be of different color?  The decorations and possible effects should be outside of the stage, shouldn't they?
>
> Turning the scene to undecorated would just make the math easier, without having to count in the window decorations which almost certainly would be of different color.
> 
> As for the margin thing, I had to do a bit of history searching and I came back with this old PR: https://github.com/openjdk/jfx/pull/1242
> 
> I think when writing the above my blurry memory recalled one of the attempts at solving this which involved just skipping the 1px margin of the Stage on HiDPI systems. I agree, it should be fine just going through the entire Stage.

Actually a small correction, using `VisualTestBase` already assumes the Stage we get is undecorated. So no changes need to be made there.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1804#discussion_r2179768493