Two classifiers, two truths

A person taps a text field on a phone and the keyboard rises, and the field they were about to type in is no longer on the screen. Everything a test would check has survived. The input element is still in the document, it is still the focused element, and its text cursor is still blinking somewhere below the visible edge of the application. This is the failure a review packet we read this year was commissioned to explain, and the explanation is not a bug in the ordinary sense. Nothing miscomputed.

Two different parts of the application decide, independently, whether a keyboard is open. One of them accepts a rich set of evidence, the height the screen had before anything moved, the physical screen height, and the fact that a focused editor may own the contraction, and during the failure it decided that the keyboard was open. The other accepts a narrower set, the current heights and whether it had recently believed a keyboard was open, and on the platform’s newer behavior, where both heights shrink together, that narrower evidence cannot distinguish a keyboard from any other reason a window got smaller. It decided the keyboard was closed and published a usable height that was roughly double the space the outer container had actually taken. The container beneath it did exactly what it was built to do and pinned the text input to the bottom of that larger region, which put it below the visible clip, by construction, while focus stayed exactly where it was.

The mechanism is not disagreement but the missing room in which to disagree, because two independent decisions about one fact yield two answers that are never compared anywhere in the system, so nothing ever holds a contradiction, only two truths, each locally correct on its own evidence and each consumed by a different half of the layout.

The most expensive version of this on record left the planet. In 1999 the Mars Climate Orbiter was lost on arrival because ground software supplied thruster impulse in pound-force seconds while the spacecraft’s navigation expected newton-seconds. Neither program was wrong internally. Both were consistent, both were tested, and the two truths met for the first time in the atmosphere of another planet. Every subsequent recommendation in that investigation was a version of the same structural point, that the fix is not more careful agreement between two owners of one quantity but ending the situation in which there are two.

Which is why the repair here is not to give the narrower classifier better inputs. That is a behavioral fix, a promise that two components will keep saying the same thing, and it holds only until someone extends one of them under deadline. The structural fix is one authority per fact, which here means naming the single component that decides whether a keyboard is open, having it publish a usable height, and having every surface that lays out against it read the published value rather than deriving its own. Then a wrong answer is still possible and a divergent one is not, which is a strictly smaller class of failure, and the second classifier’s disappearance is the whole of the change.

Our own reading of that packet turned up a fitting coda. One of the numbers in its classifier comparison had been reconstructed from how the code says data flows rather than read off a live instrument, so the exact divergence at that instant is not fully established. It changes nothing structural. The duplication is visible in the source whether or not the two answers differed on that Tuesday, and a system with two authorities for one fact is already broken in the way that matters, quietly, until the day it takes something away from the person holding the phone. Build one place where the truth lives, and disagreement becomes a thing you can find.