Commits on Mar 20, 2023

1. Check if suspensey instance resolves in immediate task …

   When rendering a suspensey resource that we haven't seen before, it may
   have loaded in the background while we were rendering. We should yield
   to the main thread to see if the load event fires in an immediate task.
   
   For example, if the resource for a link element has already loaded, its
   load event will fire in a task right after React yields to the main
   thread. Because the continuation task is not scheduled until right
   before React yields, the load event will ping React before it resumes.
   
   If this happens, we can resume rendering without showing a fallback.
   
   I don't think this matters much for images, because the `completed`
   property tells us whether the image has loaded, and we currently never
   block the main thread for more than 5ms at a time (for now — we might
   increase this in the future). It matters more for stylesheets because
   the only way to check if it has loaded is by listening for the
   load event.
   
   This is essentially the same trick that `use` does for userspace
   promises, but a bit simpler because we don't need to replay the host
   component's begin phase; the work-in-progress fiber already completed,
   so we can just continue onto the next sibling without any
   additional work.
   
   As part of this change, I split the `shouldSuspendCommit` host
   config method into separate `maySuspendCommit` and `preloadInstance`
   methods. Previously `shouldSuspendCommit` was used for both.
   
   This raised a question of whether we should preload resources during
   a synchronous render. My initial instinct was that we shouldn't,
   because we're going to synchronously block the main thread until the
   resource is inserted into the DOM, anyway. But I wonder if the browser
   is able to initiate the preload even while the main thread is blocked.
   It's probably a micro-optimization either way because most resources
   will be loaded during transitions, not urgent renders.

   

   acdlite committed Mar 20, 2023
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