RFR: 8342703: CSS transition is not started when initial value was not specified

[Marius Hanl]

On Mon, 21 Oct 2024 15:32:28 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> When the initial value of a styleable property is not specified in a stylesheet, no transition is started:
> 
>     .button {
>         transition: -fx-opacity 1s;
>     }
>     
>     .button:hover {
>         -fx-opacity: 0.5;
>     }
> 
> The expected behavior is that a transition is started in this case, since the default value of `-fx-opacity` is 1.
> 
> The reason for this bug is that `StyleableProperty` implementations do not start a CSS transition when the value is applied for the first time. The intention behind this is that a node that is added to the scene graph should not start transitions. CSS transitions should only be started _after_ the node has been shown for the first time.
> 
> The logic to detect this situation is currently as follows:
> 
>     // If this.origin == null, we're setting the value for the first time.
>     // No transition should be started in this case.
>     TransitionDefinition transition = this.origin != null && getBean() instanceof Node node ?
>         NodeHelper.findTransitionDefinition(node, getCssMetaData()) : null;
> 
> 
> However, this does not work. When no initial style is specified in the stylesheet, `this.origin` will not be set, and thus no transition will be started even after the node has been shown. The new logic works like this:
> 
> A `Node.initialCssState` flag is added. Initially, this is `true`. Manually calling `applyCss` or similar methods will not clear this flag, as we consider all manual CSS processing to be part of the "initial CSS state". Only at the end of `Scene.doCSSPass` will this flag be cleared on all nodes that have expressed their interest. This mechanism ensures that a node will be eligible for CSS transitions only after the following conditions have been satisfied:
> 1. The node was added to a scene graph
> 2. CSS processing was completed for a scene pulse

modules/javafx.graphics/src/main/java/javafx/scene/Scene.java line 607:

> 605:      * CSS state (see {@link Node#initialCssState}.
> 606:      */
> 607:     private final Set<Node> clearInitialCssStateNodes = Collections.newSetFromMap(new IdentityHashMap<>());

why not use a normal `HashSet`? What advantage has this approach?

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PR Review Comment: https://git.openjdk.org/jfx/pull/1607#discussion_r1856527311