Mount continuity is not usability

This corpus argues, at length and without much hedging, that a guarantee resting on shape beats a guarantee resting on a promise. The argument holds. What follows is where it stops, and publishing the limit is part of holding the position honestly, because an argument that never says what it does not cover gets over-read by the people who believe it most.

A review packet we read this year records a repair that worked. An earlier defect had destroyed the text field when the keyboard opened, taking focus, the draft, and the session state with it. The fix kept the subtree alive through the contraction, and the regression test written alongside it asserts exactly the right things. The original node stays connected to the document. The focused element is still the focused element. There is no remount. The test passes, and it passed on the morning a person could not type into that field.

The geometry, measured live after the outer container had been corrected experimentally, is the whole of the finding. The visible shell was three hundred and seventy-seven pixels tall. The content region inside it still believed it had seven hundred and fourteen, so the region holding the text field was pinned to the bottom of a space more than twice the height of the space anyone could see, putting the input at six hundred and twenty-four to seven hundred and fourteen and the text field itself at six hundred and thirty to six hundred and fifty-eight. Below the clip, by construction. And the field was still connected, still focused, still the document’s active element, with the cursor blinking somewhere nobody would ever find it.

The mechanism is that a continuity check answers whether the same element is present and focused, and no combination of those answers describes where it is, so an acceptance contract assembled entirely from continuity assertions is fully satisfied by a layout that has put the element off the screen.

The existing test could not have caught it, and the reason repays a minute. It contracts the visual viewport from eight hundred and forty-four to five hundred and twelve while holding the reported window height at eight hundred and forty-four, which is the ordinary case where the two disagree and the browser tells you plainly that a keyboard arrived. The failure needs the other case, where both heights shrink together and the contraction is indistinguishable from any other reason a window got smaller. But even given the right case, the test would still have passed, because it never measures the input’s rectangle against the visible one. It was not written to. That was never among the things it was asked to know. The same packet documents a second and separate finding about two components deciding one fact independently, which this corpus treats elsewhere and this piece does not repeat.

The packet states its own correction in a sentence we have not improved on. The acceptance contract must include on-screen geometry, not only mount continuity.

The standards bodies got there the same way, by shipping the incomplete version first. When the Web Content Accessibility Guidelines reached version 2.2 in October 2023, one of the new success criteria was Focus Not Obscured, requiring that a component receiving keyboard focus not be entirely hidden by content the author put on the page. Every earlier criterion in that family asked whether focus was visible in the sense of having an indicator, which is a property of the element, and a decade of practice was needed to notice that an element can carry a perfect focus indicator underneath a sticky footer. The correction is not that the earlier criteria were wrong. It is that they described the element and not the encounter.

So the honest form of our own thesis needs a clause. Structural over behavioral is a claim about what a guarantee rests on, and it is not a claim that any particular structural assertion is the right one. A shape can be asserted with total precision and be the wrong shape. Node identity, focus retention, and mount continuity are all structural, all cheap to verify, all genuinely worth having, and not one of them is the property the person holding the phone depends on, which is that the thing they are typing into is inside the rectangle they can see. The corpus that argues hardest for structure is the right place to say where structure has to be pointed, and pointing it at the encounter rather than the element is how the argument grows up.