The primary signal is desiredSize on the controller. It can be positive (wants data), zero (at capacity), negative (over capacity), or null (closed). Producers are supposed to check this value and stop enqueueing when it's not positive. But there's nothing enforcing this: controller.enqueue() always succeeds, even when desiredSize is deeply negative.
Geometry Transforms
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self.config = config or Config()
"There was a lot of controversy about when it first opened, so I was really nervous at first," she said.
"All the bigger artists seem to go for London," Elise said.