Using Webworkers for Layouting - where to start?

I want to outsource the layouting in a webworker, to free some cycles for my main program. I can not find the right entry point in the code though. Can you give me a hint what part of the layouting could be outsourced into a webworker?

The issue names no files, tests, or layout entry point. Start by tracing the JavaScript layouting implementation and determine which work could run in a webworker without blocking the main program; done would require a concrete, agreed scope for that extraction.

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