Re: [w3ctag/design-reviews] navigator.scheduling.isFramePending (#415)

@hober @plinss and I are looking at this again at our Cupertino face-to-face meeting, and we pulled in @smfr since he's here.

One of the things that came out of the discourse thread mentioned in the previous comment that we discussed a bit is that it sounds like the intent (although that isn't clear from the explainer, and probably should be!) is that `isFramePending` should become true as a result of some vsync trigger that comes from the OS, not as a result of the JS that dirties the DOM or style that ends up triggering the need for a repaint.  This actually makes me a little bit more worried about interoperability here:  it's not clear to me how interoperably those OS signals are defined, whether they behave in different ways on different platforms, e.g., how much does the time between when `isFramePending` becomes true and when the main-thread rendering is done vary between platforms?   how much does this differ for a page in a background tab?  also, when does `isFramePending` become *unset*?

We also talked a little bit about whether there are `:visited` link history-sniffing attacks that would result from this.  My assertion was that it didn't seem like a big deal because we should be attempting to audit from the side of not having different performance characteristics whether or not links are visited.  But @smfr pointed out that there could be optimizations (e.g., those based on display-list diffing) that might need to be disabled in that case, and which could (depending on how it's defined) affect whether `isFramePending` is true.

It might help the review progress a bit if the explainer tried to flesh out the answers to these questions a little bit more.  And if you do update the explainer in response to this, please ping us in this issue so we know to take another look at it.

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