Translucent window backdrops for stages

[Martin Fox]

I’ve submitted a PR (#2048) for enabling system backdrops in JavaFX windows. This allows JavaFX to tap into the translucent window backgrounds common on macOS and Windows 11. Since these effects require real-time blurring of the content behind the window they can only be rendered by the platform; JavaFX then alpha composites its content on top.

My goal was to come up with a useful set of backdrops which exhibit similar behavior across platforms. For example, all of the backdrops respond to the window’s color scheme preference when determining their dominant color and consistently obey accessibility settings like reduceTransparency.

I was not trying to support platform-specific backdrops like Glass on macOS 26. I don’t think Glass Regular is intended for top-level windows and Glass Clear is a special effect that only makes sense for something like the dock. (I also have some questions about the longevity of these effects. The UNIFIED stage style is defunct for a reason; glass and brushed metal window backgrounds don’t age well.)

The PR doesn’t support Linux. Blurred window backgrounds have been kicking around on Linux for some time but there hasn’t been a standardized API for requesting them. Wayland has just started the process of standardizing a background effect extension protocol and I have no idea what the deployment schedule might be.

The PR doesn’t support Windows 10. On that platform we need to program to a lower-level API and implement the entire backdrop effect ourselves responding to all the necessary window state changes and platform accessibility preferences. That API also requires a custom ViewPainter and altering Prism to deliver pixels to the window through a new path. The testing footprint for this would be large.

These backdrops cover the entire window which limits their utility for transparent stages. If you style, say, a menu with rounded corners and a drop shadow and then try to place a backdrop behind it the backdrop will extend behind the shadow and ignore the corner rounding. I think this is an inherent limitation. Even on a platform like macOS where we can shape the backdrop it would be tricky to align a shaped backdrop drawn by the platform with edge effects drawn by JavaFX (though to be honest I haven’t prototyped this). Unfortunately this means we can’t support the obvious use case of adding a translucent backdrop to a CSS-styled menu that uses a drop shadow.

I’m open to suggestions on how this feature should interact with CSS. Initially I thought that a stylesheet should be able to specify a backdrop but currently the CSS code only targets nodes for styling. It was also suggested that there should be media queries related to backdrops but I’m not sure what those would be.

The end result is a simple API which allows a developer to choose one of three backdrops for a stage. This needs to be expanded to future-proof it but before that can happen we need to sort out top-level issues like whether Win10 support is required, whether we need to support platform specific backdrops like glass on macOS, and how backdrops should interact with CSS. Thoughts, questions, and feedback welcome.

Martin