RFR: 8345348: CSS media feature queries [v21]

[John Hendrikx]

On Wed, 7 May 2025 13:06:50 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> `NullCoalescingPropertyBase` (used in `ScenePreferences`) is initially disconnected. It will set its initial value to the value is reads from `PlatformPreferences`, but it will not subscribe to change notifications. The reason for this is to prevent a scene that is created on a background thread from immediately receiving change notifications on the FX thread, which could not only potentially corrupt the property state (because they're not concurrently thread-safe), but also mess up the application state.
>> 
>> When the scene is shown, `NullCoalescingPropertyBase` will subscribe to change notifications from `PlatformPreferences`, because now we're safely on the FX thread. The code for that is in ScenePreferences:L142.
>
>> I think it (creating things in a background thread) was a mistake in the first place, but there seems to be zero desire to change it
> 
> Your objection is noted, but we are not going to relitigate this. JavaFX explicitly allows creation and modification of a scene graph on a thread other than the JavaFX application thread up to the point before that scene is attached to a window.
> 
> This means that we do need to be careful in our implementation, especially avoiding anything that might run on the application thread prior to the scene becoming "live". Sometimes that means deferring the adding of listeners as was recently done for charts. It sounds like that's what is being done here by deferring the subscription to platform preferences until the scene is shown.

@kevinrushforth ... what about manipulating a Scene that was at some point attached to a Window?  I mean, I've built a solution specifically for just-in-time registration **and** unregistration of listeners.  So IMHO when a Scene is detached or the window is no longer showing (basically any time it doesn't have a peer) then listeners need to detach as well (and re-attach when that situation changes again).

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PR Review Comment: https://git.openjdk.org/jfx/pull/1655#discussion_r2077627120