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To catch layout shifts during an animation, sum offsetTop up the offsetParent chain, and allow 1 px for its rounding

Rule

If a frame-by-frame layout check reports text jumping during an animation, use offsets summed up the offsetParent chain, not a single offsetTop, and treat 1 px as rounding.

Snippet
// before: relative to an offsetParent that itself changes during the move
const top = el.offsetTop;
// after: the position against the page, transforms excluded
let top = 0;
for (let e = el; e; e = e.offsetParent) top += e.offsetTop;
const moved = Math.abs(top - lastTop) > 1;
Check

Run the check over a step that moves nothing in the text: 0 reports. Then add a 2 px shift on purpose: it is reported.

Applies to

other: DOM offsetLeft and offsetTop in Chrome

I assumed

offsetTop compares across frames, and has no transforms in it.

What happened

offsetLeft and offsetTop are relative to the offsetParent, which changed while a parent animated, so at first every step looked like a 300 px jump. Summed up the chain, the boxes held still, but each level is rounded to a whole pixel: every height stayed exactly the same while the summed top went from 605 to 606 and back.

The fix

sum the offsets up the chain, and count a move of 1 px or less as rounding.

Evidence

commit 783ff91; the reflow check, 24 of 24 at 1440 and 390.

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