> This PR enables translucent window backdrops for JavaFX stages on macOS and 
> Windows 11. Since we’re reliant on the operating system for these effects 
> (they typically require real-time blurring of the desktop) I needed to flesh 
> out a fairly complete prototype to sort out the API. I will start a 
> discussion about the API on the mailing list.
> 
> There’s a crude manual test for trying out the different backdrop materials.
> 
>       java @build/run.args -Djavafx.enablePreview=true 
> tests/manual/stage/BackdropTest.java
> 
> You’ll want to drag the windows around to avoid having them overlap each 
> other since they’re all created in the center of the screen. For windows 
> without title bars you can click anywhere on the background to drag the 
> window except for TRANSPARENT stages on Windows which are a bit tricker to 
> get a hold of; try to click on a text label.
> 
> If you create an UNDECORATED stage on Windows the backdrop won’t be 
> translucent initially. This can be corrected by changing the stage’s color 
> scheme. This is an OS bug that I haven’t found a workaround for.
> 
> The changes on Windows 11 are minimal since we’re just invoking an OS feature 
> by calling DwmSetWindowAttribute. I did need to make two small changes to the 
> D3D9 Prism code to ensure that the swap chain and back buffer support an 
> alpha channel so JavaFX can composite its content on top of the backdrop. 
> This is the same way the old UNIFIED stage style worked before it became 
> unreliable (see [JDK-8154847](https://bugs.openjdk.org/browse/JDK-8154847)).
> 
> On macOS I moved the GlassHostView so it’s now a permanent part of the 
> NSWindow. For some time the host view has been a remnant left over from an 
> older approach to implementing fullscreen. Now it serves as a common parent 
> for the NSVisualEffectView that provides the backdrop and the GlassView3D 
> that contains the JavaFX content. Making it the permanent contentView of the 
> NSWindow simplifies some code.
> 
> To validate the API I did prototype this for Windows 10 (thanks @mstr2!). 
> Well, I prototyped this using DirectComposition so it should work on Win10 
> but I can't test Win10 myself. Using DirectComposition is much more involved 
> so I shelved that implementation for now but it does inform the API. It’s the 
> reason the backdrop needs to be specified before the Java window is shown and 
> the platform window created.
> 
> ---------
> - [x] I confirm that I make this contribution in accordance with the [OpenJDK 
> Interim AI Policy](https://openjdk.org/legal/ai).

Martin Fox has updated the pull request incrementally with one additional 
commit since the last revision:

  Fixed some bugs in how window flags are set up.

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/2048/files
  - new: https://git.openjdk.org/jfx/pull/2048/files/7fbdf2b7..49901264

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2048&range=09
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2048&range=08-09

  Stats: 8 lines in 1 file changed: 5 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2048.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2048/head:pull/2048

PR: https://git.openjdk.org/jfx/pull/2048

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