SchedulerDOM falls back to globalThis if window is undefined

[bvaughn]

In response to user feedback from @rickhanlonii, I recently made a small change to our scheduling profiler so that it stores lane labels in addition to numeric values (#20808).

While re-building the profiler UI to add the newly available info, I found that it throws when importing profiling data because of references to window (e.g. window.setTimeout) inside of SchedulerDOM. (Some of the scheduling profiler code runs in a web worker and window isn't defined there.)

This PR adds a fallback to globalThis if window is not defined. It prefers window if available to minimize change. We could perhaps also workaround this by falling back to something else e.g. self but globalThis seemed better.

Thoughts?

[sizebot]

Comparing: c62986c...0182034

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	122.36 kB	122.36 kB	=	39.41 kB	39.41 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	128.94 kB	128.94 kB	=	41.47 kB	41.47 kB
facebook-www/ReactDOM-prod.classic.js	=	404.67 kB	404.67 kB	=	75.15 kB	75.15 kB
facebook-www/ReactDOM-prod.modern.js	=	393.02 kB	393.02 kB	=	73.24 kB	73.24 kB
facebook-www/ReactDOMForked-prod.classic.js	=	404.68 kB	404.68 kB	=	75.15 kB	75.15 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/scheduler/cjs/scheduler.development.js	+1.68%	16.45 kB	16.72 kB	+2.44%	4.63 kB	4.74 kB
oss-stable/scheduler/cjs/scheduler.development.js	+1.68%	16.45 kB	16.72 kB	+2.44%	4.63 kB	4.74 kB
facebook-react-native/scheduler/cjs/Scheduler-dev.js	+1.67%	16.49 kB	16.77 kB	+2.53%	4.62 kB	4.74 kB
facebook-www/Scheduler-dev.classic.js	+1.19%	23.76 kB	24.04 kB	+1.80%	6.35 kB	6.46 kB
facebook-www/Scheduler-dev.modern.js	+1.19%	23.76 kB	24.04 kB	+1.80%	6.35 kB	6.46 kB
facebook-react-native/scheduler/cjs/Scheduler-prod.js	+0.93%	10.14 kB	10.23 kB	+1.11%	2.61 kB	2.64 kB
facebook-react-native/scheduler/cjs/Scheduler-profiling.js	+0.93%	10.14 kB	10.23 kB	+1.11%	2.61 kB	2.64 kB
facebook-www/Scheduler-prod.classic.js	+0.88%	11.41 kB	11.51 kB	+0.94%	2.87 kB	2.89 kB
facebook-www/Scheduler-prod.modern.js	+0.88%	11.41 kB	11.51 kB	+0.94%	2.87 kB	2.89 kB
facebook-www/Scheduler-profiling.classic.js	+0.66%	15.11 kB	15.21 kB	+0.79%	3.56 kB	3.59 kB
facebook-www/Scheduler-profiling.modern.js	+0.66%	15.11 kB	15.21 kB	+0.79%	3.56 kB	3.59 kB
oss-experimental/scheduler/cjs/scheduler.production.min.js	+0.65%	4.33 kB	4.36 kB	+0.94%	1.82 kB	1.83 kB
oss-stable/scheduler/cjs/scheduler.production.min.js	+0.65%	4.33 kB	4.36 kB	+0.94%	1.82 kB	1.83 kB
oss-stable/react/umd/react.production.min.js	+0.35%	10.99 kB	11.03 kB	+0.43%	4.43 kB	4.45 kB
oss-experimental/react/umd/react.production.min.js	+0.32%	12.04 kB	12.08 kB	+0.55%	4.72 kB	4.74 kB
oss-stable/react/umd/react.profiling.min.js	+0.30%	13.21 kB	13.25 kB	+0.44%	4.98 kB	5.00 kB
oss-experimental/react/umd/react.profiling.min.js	+0.27%	14.26 kB	14.30 kB	+0.32%	5.27 kB	5.29 kB
oss-stable/react/umd/react.development.js	+0.27%	103.79 kB	104.07 kB	+0.41%	25.70 kB	25.81 kB
oss-experimental/react/umd/react.development.js	+0.27%	106.05 kB	106.33 kB	+0.40%	26.12 kB	26.22 kB

Generated by 🚫 dangerJS against 0182034

[gaearon]

Is this fixing a regression, or adding a feature? In other words, did it not throw before?

[bvaughn]

I haven't re-built and run the scheduling profiler in a while. My guess is that this broke with #20025 and we just hadn't noticed.

[rickhanlonii]

I think before we were probably using the equivalent of SchedulerNoDOM, and this change now uses SchedulerDOM: https://github.com/facebook/react/pull/20025/files#diff-1b2464331bec5a14f0c1db975b4ea10aeaa2c4dccdec3c135a1d96faf0a1e57fL34

Probably the right call, but that's why the changes broke. We're using the DOM scheduler in an non-DOM environment. If we land this, we should probably rename this scheduler.

[bvaughn]

I don't care about this specific fix. I was thinking of SchedulerDOM as SchedulerBrowser so this seemed to make sure. If the "DOM" bit is significant, then okay. I can close this.

I'll dig in more to find out how the scheduling profiler ended up in this code path.