I don't think the size of the framebuffer is an issue, it's the viewport size restriction. You can use a smaller framebuffer and appropriate viewport and scissor settings to render just the zoomed-in part of your scene into the framebuffer.
I didn't know that. That's quite odd, and completely unusable for WebGL because there are limits to the size you can set a framebuffer to as well as that rendering to ridiculous resolutions would become very slow, very quickly.
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