Apparently the first number this happens to is 800002 (Decimal.round(800002).toNumber() is 800001.9999999999).

Specifically, this happens because

1.00968*1e6
1009679.9999999999

So we can't trust that an integer turned into mantissa*exp format and then having the reverse operation done remains an integer after a certain point. Cool.

1:53 PM] Patashu [10^^e308 EP]: I guess you could always do the same VeryNearlyRound() that add does
[1:53 PM] Patashu [10^^e308 EP]: (mult by 1e14, round, div by 1e14)

floor and ceil are also busted for similar reasons
the root cause is toNumber though

ah, it's because I have a line like

if (Math.abs(resultRounded - result) < ROUND_TOLERANCE) {

and ROUND_TOLERANCE is 1e-10, and this is when it starts being hit

yeah OK, now I remember why this is fundamentally tricky to fix.
basically, after we make our Decimal and it's split into mantissa and exponent, we don't know if it used to be an integer or very close to one. setting a 'this is meant to be an integer' flag is out of the question. (though ironically, break_eternity.js with its layer system has this for free!)

we can set the 'it looks close enough to an integer, we'll say it's an integer when we call toNumber()' threshold higher or lower than 1e-10, but it's fundamentally a problem of tradeoffs.

as a workaround, you can round AFTER toNumber. but that won't fix one of floor/ceil sometimes being wrong, if you needed that.

but, let me try 1e-9 and see how long it works for.

EDIT: doesn't help for very long. I think this might just be one of those 'this is a limitation of the format, beware' things.

going to close as 'fundamentally unsolvable; implement the fix you want yourself'