Please consider adding DOM events triggered or functions called within the browser input to integrate better with vMix; a good starting point for these would be being able to trigger animations within Javascript on transition; so:

• TransitionIn
  
• TransitionOut
  

This would allow specifically built HTML pages to react to activities from vMix giving greater flexibility for building animations with JavaScript or webXR.

The CasparCG HTML Producer does something like this with it's play, next, stop and update functions available within the browser DOM that are triggered from the software, allowing for very flexible and deeper intergration between the browser and production tools.

Edit: Additional Information on other products implementing similar features:

• GitHub - k4kfh - CasparCG Javascript Function Example
  
• Github - DGCK81LNN - OBS Custom Browser Events Plugin
  

+1

+1. This would solve a long-term problem for us.

Knowing the context of the transition event would be super useful in my workflows too! This could be as simple as providing a context to the transitionIn and Out triggers I've suggested that would contain details of the mix and if it was transitioned to/from preview or program for instance.

Full tally probably should be done directly by the upstream management program rather than individual content sources which really only should get fed info about themselves for simplicity (and the option is always there for polling the API).

On that polling note though - polling is less than ideal when you want a 250-500ms animation in that you want triggered immediately on take and even more time critical than polling could feasibly produce for a transition out.