Ah, someone reported this to me today, but I must admit I have no idea how to address the issue. Currently the canvas animations are stopped when out of context, but yeah they have to be loaded.
As an aside, WebGPU doesn't have this issue or at least has it less. For one, WebGPU can use a single device to render to multiple canvases, something WebGL can't. Another is that WebGPU is mostly stateless making it easier for both the user and the browser.
> WARNING: Too many active WebGL contexts. Oldest context will be lost.