Wrench deadlocks when a transaction contains scrolling operations.

[nical]

For some reason wrench does not wake the event loop up if there is a scrolling operation in a transaction.
see

webrender/wrench/src/main.rs

Line 295 in d3f72e1

if !scrolled {

That's kinda confusing and error prone. This was probably done to avoid taking the snapshot in the middle of the updates when several things scroll, but we can better support this type of situation by grouping all scrolling updates inside the same transaction.

If we need anything more complicated, then wrench should do what gecko does: explicitly wait for specific epochs to be rendered before taking the snapshot.

[glennw]

@nical Is this resolved now? (via #2410)

[nical]

Yep!