Update: Apple got back to me! It turns out that my code had a silent bug: my conformance to the Equatable protocol wasn't considering the current color of the rectangles drawn on screen, and one of the performance improvements in macOS Sequoia/iPadOS 18 is that ForEach elements don't re-draw if they aren't changed.
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Update: Apple got back to me! It turns out that my code had a silent bug: my conformance to the Equatable protocol wasn't considering the current color of the rectangles drawn on screen, and one of the performance improvements in macOS Sequoia/iPadOS 18 is that ForEach elements don't re-draw if they aren't changed.