Note how nowhere in the code `canvasInRendering.get()` is ever called, and that this structure is really only used to store references to `<canvas>` DOM elements.
The reason for this being a `WeakMap` is probably because at the time we weren't using `core-js` polyfills yet, and since there already existed a manually implemented `WeakMap` polyfill it was probably simpler to use that.
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