why-is-dom-transaction-hard

gistfile1.md

mutateElementToAffectOffsetTop(someElement);

// (1) We don't know the value of someElement.offsetTop here because we compute it lazily.

var y = (new DOMTransaction(function () {
    // (2) We need to remember the "old value" of someElement.offsetTop here.
    mutateElementToAffectOffsetTop(someElement);
    return someElement.offsetTop; 
})).execute();

If DOMTransaction were to preserve the old value, then y needs to be identically equal to the value of someElement.offsetTop at (1). However, since nobody had asked for the value in (1), we're forced to pre-compute the value at (2). It's worse than that. mutateElementToAffectOffsetTop might be using the old value of other computed CSS values in which case we have to pre-compute all the property values that could be queried by the script at (2).