test(root): graduate the autoscroll acceptance point

[mobeenabdullah]

The canvas autoscrolls. It has all along. Plan 04's B-8 is recorded as "not built" and that is wrong.

How it was hiding

createPocChromeReader.canvasScrollTop() threw CanvasCapabilityError rather than measuring, and the acceptance case asserted that throw before marking itself test.fail. A refusal cannot become a red, so the case would have passed forever whether or not the capability existed.

This is the second acceptance point with that exact shape — the first was B-7's invalid-target reader, graduated in #847. Both were recorded as product shortfalls when the evidence was a reader declining to look.

Measured, before any code was changed

A throwaway probe held a drag at the canvas's bottom edge and read every candidate scroller:

BEFORE  frameDoc {scrollTop: 0,    scrollHeight: 6003, clientHeight: 1041}
        host     {wrapScrollTop: 0, wrapScrollHeight: 1041, wrapClientHeight: 1041}
AFTER   frameDoc {scrollTop: 3192, ...}
DELTA   frameDoc: 3192   wrap: 0

dnd-kit ships AutoScroller in defaultPreset, and getElementFromPoint explicitly recurses into an iframe's contentDocument with adjusted coordinates — so the cross-document canvas never defeated it.

Note which element moves. The host wrapper around the iframe has overflow: auto and is the obvious thing to watch; it stays at 0, because the iframe is sized to it and the document that overflows is the one inside. A reader pointed at the wrapper reports "no autoscroll" about a canvas that is scrolling.

What the reader returns now

canvasScroll(): Promise<{ top, max }> replaces canvasScrollTop(). The bound is not decoration:

Two equal offsets are satisfied by a scroll that stalled for any reason, including one that never started.

"Stops at the bounds" is the requirement, so the case now asserts the scroll reaches max, does not exceed it, and stays there. Without max, the stopping half was satisfied by a canvas that never moved.

Two mechanics the old case got wrong

• It walked the pointer down in fixed 40px steps. That runs off the viewport before dwelling anywhere, which reads as the canvas ignoring the gesture. It now goes to the edge via the shared stepped transport and dwells there.
• A perfectly still hold produces no further signal. dnd-kit recomputes scroll intent from the drag position, so the dwell moves a pixel each tick. A motionless hold can measure a canvas that simply stopped being told anything.

canvasBottomEdge() is on the driver rather than measured in the suite, for the same reason canvasCentre() is: a replacement canvas that is not an iframe still has a bottom.

Break verified at the source

Removing AutoScroller from the provider's plugin list and rebuilding fails the case with:

Error: autoscroll must engage while the pointer rests near an edge
Expected: > 0
Received:   0

Restored → passes. The capability was removed at its origin, not stubbed in the harness, so the red is about the canvas rather than the reader.

Verification

• The graduated case: passes, no test.fail.
• Full canvas suite: 93 passed, 0 failed, and B-8 is no longer among the expected failures.
• check-types + lint green for @nextlyhq/e2e, turbo --force.

Consequence for the plan

page-builder-master-plan.md §5c and the focus tracker both list B-8 under "Not built". It is built. Anyone sizing plan 04 from that list is over-counting, and B-8 should not be scheduled as work.

No changeset: test-only.

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• e2e/tests/canvas/acceptance.spec.ts
• e2e/tests/canvas/driver.ts
• e2e/tests/canvas/poc-driver.ts

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[mobeenabdullah]

@codex please review this PR

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commit: 35f77d6

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[mobeenabdullah]

@codex please review this PR

[mobeenabdullah]

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Review in progress

Reviewing PR #868 (test(root): graduate the autoscroll acceptance point) at 35f77d6e.

• Phase 0 — Pre-flight: PR open, headRefOid matches 35f77d6e (no supersede), base 6b154cd3
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[chatgpt-codex-connector]

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Reviewed commit: 35f77d6eae

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