Implement CSS Transition Events (transitionend)

[mbrubeck]

https://drafts.csswg.org/css-transitions/#transition-events

Aside from web compatibility, this is also important for writing reliable automated tests that use transitions.

[saurvs]

I'm working on this.

[jdm]

For what it's worth, places to start looking for the actual transition-related code:

• servo/components/style/matching.rs

  Line 451 in a9eb08d

  // Finish any expired transitions.

  seems to be code that deals with determining if an animation is finished
• we would need to add a way to send messages to the script thread (maybe like the animations_sender member of SharedStyleContext) with a pipeline, node, and event type - this message would cause a new event of the desired type to be dispatched at the target node that is inside the document associated with the given pipeline

• servo/components/style/animation.rs

  Line 933 in a9eb08d

  pub fn start_transitions_if_applicable<C: ComputedValues>(new_animations_sender: &Mutex<Sender<Animation>>,

  might be a good place to a add similar message for starting a transition (this might not be necessary; I haven't read the specification)

[saurvs]

The spec says that only the completion of a css transition generates a corresponding dom event.

[jdm]

Unassigning under the assumption that this is not being worked on given the lack of response in #10393.

[jdm]

@saurvs indicated that they're continuing to work on this.

[KiChjang]

@saurvs Is this still being worked on?

[saurvs]

@KiChjang I was busy for a while. I'd like to finish it now.

[KiChjang]

@saurvs What's the status of this?

[saurvs]

@KiChjang Sorry, I'd couldn't figure out the surrounding code for sending the event, and can't make progress. I hope my code about the DOM of the transition event #10393 is useful.