[css-shared-element-transitions-1] Timeout for transition preparation

[FremyCompany]

What happens if, through some error in programming, the prepare() promises never resolves?

The spec says that the page never re-renders anything on the screen, but I would assume this is a drastic error case for a transition effect (which is usually just a nice-to-have).

It would probably make sense to automatically abandon the transition if it takes too long to prepare. There could be a default timeout, and maybe a parameter to supply a custom timeout in case authors expect the transition to take a long time (but this seems counter-productive).

[khushalsagar]

If the promise returned by the updateDOM callback doesn't resolve within a UA defined timeout, the browser stops suppressing rendering and abandons the transition. This case is already spec'd. Please feel free to reopen in case I misunderstood.