Commits on Jul 21, 2021

1. Fix: LegacyHidden should not toggle effects …

   LegacyHidden is a transitional API that we added to replace the old
   `<div hidden={true} />` API that we used to use for pre-rendering. The
   plan is to replace this with the Offscreen component, once it's ready.
   
   The idea is that LegacyHidden has identical behavior to Offscreen except
   that it doesn't change the behavior of effects. (Which is basically how
   `<div hidden={true} />` worked — it prerendered the hidden content in
   the background, but nothing else.) That way, while we're rolling this
   out, we could toggle the feature behind a feature flag either for
   performance testing or as a kill switch.
   
   It looks like we accidentally enabled the effects flag for both
   Offscreen _and_ LegacyHidden. I suppose it's a good thing that nobody
   has complained yet, since we eventually do want to ship this
   behavior everywhere?
   
   But I do think we should remove it from LegacyHidden, and roll it out by
   gating the component type in the downstream repo. That way if there's an
   issue related to the use of LegacyHidden, we can disable that without
   disabling the behavior for Suspense boundaries.
   
   In retrospect, I might have implemented this as an unstable prop on
   Offscreen instead of a completely separate type — though at the time,
   Offscreen didn't exist. I originally added LegacyHidden to unblock the
   Lanes refactor, so I could move the deprioritization logic out of the
   HostComponent implementation.
   
   Not a big deal since we're going to remove this soon. The implementation
   is almost the same regardless: before disconnecting or reconnecting
   the effects, check the fiber tag. The rest of the logic is the same.

   

   acdlite committed Jul 21, 2021
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