Description
Relates to PR #12083
Currently we double-invoke component constructor, render, static getDerivedStateFromProps and setState reducer functions if the debugRenderPhaseSideEffects feature flag is on. We should also do this if we're inside of a StrictMode subtree.
However, it is likely that the new StrictMode element will be present in production apps, so I believe it is important that it not negatively impact performance.
Strict warnings (like all warnings) only impact dev mode, but up until now the double-invoked lifecycles impact both dev and prod. This is actually a good thing at Facebook, since it helps us identify potential bugs in production and we can easily limit it to only impact developers, but I think it would complicate the story for external users because of the negative performance impact.
I propose that we continue to observe the debugRenderPhaseSideEffects flag for dev and prod (since this impacts Facebook only) but only double-invoke for "strict" mode in dev.
Enabling this behavior greatly complicates our incremental rendering tests (see below so I will also be converting these to be internal so they can explicitly override the feature-flag before running. I think this is okay since the tests that break all use the noop-renderer which is not published to NPM.