Commits on May 1, 2020

1. Unhide Suspense trees without entanglement …

   When a Suspense boundary is in its fallback state, you cannot switch
   back to the main content without also finishing any updates inside the
   tree that might have been skipped. That would be a form of tearing.
   
   Before we fixed this in facebook#18411, the way this bug manifested was that a
   boundary was suspended by an update that originated from a child
   component (as opposed to props from a parent). While the fallback was
   showing, it received another update, this time at high priority. React
   would render the high priority update without also including the
   original update. That would cause the fallback to switch back to the
   main content, since the update that caused the tree to suspend was no
   longer part of the render. But then, React would immediately try to
   render the original update, which would again suspend and show the
   fallback, leading to a momentary flicker in the UI.
   
   The approach added in facebook#18411 is, when receiving a high priority update
   to a Suspense tree that's in its fallback state is to bail out, keep
   showing the fallback and finish the update in the rest of the tree.
   After that commits, render again at the original priority. Because low
   priority expiration times are inclusive of higher priority expiration
   times, this ensures that all the updates are committed together.
   
   The new approach in this commit is to turn `renderExpirationTime` into a
   context-like value that lives on the stack. Then, when unhiding the
   Suspense boundary, we can push a new `renderExpirationTime` that is
   inclusive of both the high pri update and the original update that
   suspended. Then the boundary can be unblocked in a single render pass.
   
   An advantage of the old approach is that by deferring the work of
   unhiding, there's less work to do in the high priority update.
   
   The key advantage of the new approach is that it solves the consistency
   problem without having to entangle the entire root.

   

   acdlite committed May 1, 2020
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