[cixzhang]

Maintainer impact

• People on a phone see a determinate ProgressBar move the way iOS draws it: linearly, from where the fill is drawn, at one second per whole track of change (10% to 90% takes 0.8 s). Above zero the fill is at least 16px long; at zero nothing is painted. A mouse or trackpad keeps today's eased theme timing.
• People with Reduce Motion on, on any pointer, see an indeterminate bar that no longer travels: a full-width solid bar whose opacity pulses slowly (2 s each way, from 1 down to 0.84). Without Reduce Motion the indeterminate bar keeps today's 40% eased slide on every pointer.
• Product teams: no prop, default or value change. Both behaviors are recorded as implementation gaps until built.
• Contrast: at the 0.84 floor every tone that reaches 3:1 against the track at full opacity still does, in light and dark (lowest 3.01:1, success in dark). Warning and neutral in the light default theme are below 3:1 even at full opacity (1.65–2.03:1), so no opacity floor can reach 3:1 for them; that is recorded as a gap under OQ1, the open standalone-contrast treatment.

Exact decision needed

How a determinate fill moves under a coarse primary pointer, and what the indeterminate bar shows under reduced motion.

Existing authority searched

component:ProgressBar (FR4, FR6, AR1, OQ1, OQ2), component:Skeleton (FR3, FR4), spec:AST-094 (FR1, FR2, FR4, FR13, DEC-1, DEC-5, DEC-6, DEC-15), spec:AST-108 (FR2, FR3, FR4, DEC-1), design:reduced-motion (DR1–DR3, DEC-3, content boundary), design:motion, the theme duration tokens (longest is 1300 ms), and the registered motion tables. AST-094 makes native measured timing the touch default and requires named roles, but leaves each component's presentation to its own record; the reduced-motion record leaves continuous status indicators out of scope; ProgressBar OQ2 held the reduced-motion loop open. ProgressBar's record owns both decisions.

Decision

The owner ruled on 2026-11-01:

"go with proposal for determinate"
"for indeterminate, keep original animation"
"skeleton keep current"
"for indeterminate we do need a reduced motion variant"
"it should be a solid bar that slowly pulses opacity on the fill"

Changed records: component:ProgressBar only. component:Skeleton is unchanged.

• FR4 amended: under a coarse primary pointer, the fill moves linearly from where it is drawn, for the progress.fillTrackMs role (1000 ms, UIKit's progress view for a whole-track change) times the share of the track moved; at least 16px above zero; nothing at zero. The duration and its linear curve (progress.curves.fill) are roles in the generated motion role table, and tempo scales the duration (spec:AST-094 FR4, FR13, DEC-1, DEC-5; spec:AST-108 FR3). Fine pointers unchanged; reduced motion still lands at once.
• FR6 amended: without reduced motion, today's 40% eased slide on every pointer. Under reduced motion, on every pointer, a full-width solid fill with no segment and no travel, pulsing only its opacity between 1 and 0.84, each way on the progress.reducedMotionPulseMs role (2000 ms) and the existing timing.curves.move curve; the track is unchanged. No theme duration token is near 2 s, so the pulse is a named role; as a busy loop it is on the fixed list, so tempo does not scale it. It applies design:reduced-motion DR1's no-travel rule (an opacity change in place of travel).
• AR4 added: the pulse floor of 0.84 and the contrast it keeps.
• DEC-2 added; OQ2 removed (answered).
• Current-state impact: three gaps (touch timing unbuilt; the reduced-motion slide still travels; warning and neutral below 3:1 in light).
• Verification: pending rows for coarse vs fine pointer timing, the 16px minimum, zero, the reduced-motion pulse with no travel, and contrast at the floor in both themes.

This unblocks #8225 and the Reduce Motion part of #8239; #8239's quarter-track sweep under a finger and its 0.74–1.0 pulse are not adopted. #8253 (Skeleton) is not adopted.

Rejected alternatives

• iOS Safari's quarter-track sweep for the indeterminate bar under a finger (the owner kept the 40% slide).
• Keeping a slowed slide under reduced motion: it still travels.

Claim scope and non-goals

Determinate fill timing under a coarse pointer and the indeterminate bar under reduced motion only. Skeleton, the fine-pointer determinate motion, the standalone contrast treatment (OQ1) and the tone colors are unchanged.

Evidence and implementation relationship

UIKit measurement of the progress view's animated timing (whole track 1 s, linear) and minimum fill, iOS 26.2 simulator. Contrast computed from the default theme tokens: each fill at 0.84 opacity composited over --color-background-muted on the body and surface backgrounds, light and dark. No implementation in this PR.

Writing rubric results

• R1 narration: none; the new FR4 and FR6 text, AR4, DEC-2 and the gaps are present tense about the system.
• R2 research in the record: none; the per-tone contrast table stays here, and AR4 keeps the floor and its lowest value.
• R3 evidence naming a change: none; the FR4 basis names the UIKit measurement and DEC-2.
• R4 rejected alternatives over one line or outside a decision: one, in DEC-2, one line.
• R5 pull request or issue references remaining: none in the record.
• R6 ruled open questions still listing options: none; OQ2 is answered and removed, and OQ1 is unanswered and unchanged.

Scope

• This PR contains specification records only.
• It records observable behavior and preserves equivalent internal implementations.
• Any named internal mechanism is intentionally public, names its dependent caller/system, and explains why an equivalent implementation would not satisfy the contract.
• Verification states the evidence layer and failure signal without prescribing CI job/workflow topology.
• It records one intentional durable decision, not a rescue for a separable tagalong.
• The canonical knowledge owner, affected current claims, and overlapping open work are named; any ownership collision is resolved or called out for human decision.
• Adjacent API, visual, accessibility, composition, and implementation facts remain outside scope unless this exact decision depends on them.
• Public text and artifacts contain no internal Meta context.

Validation

• pnpm install --frozen-lockfile
• pnpm check:knowledge: Knowledge templates and records are aligned.
• Prettier: clean
• git diff --check: clean

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Review status	Updated
🔄 In progress (for maintainers only)	Nov 2, 2026, 1:44 AM UTC

[cixzhang]

/approve-spec 290c3d7