The current behavior in the spec is that starting any view transition cancels the active one, which means that the following edge cases might have unexpected results:

• A capture is initiated for a document navigation and script initiates a transition before the document unloads. should we ignore document.startViewTransition() calls during a navigation initiated transition?

• On the new Document, script initiates transition before the navigation initiated transition could finish (before pagereveal). Given that the navigation initiated transition will be before the first frame of the new Document was rendered, should we ignore it and let the MPA transition go through?

@khushalsagar @bokand @vmpstr