348. A canvas asks for its next frame with what it was painted with

Date: 2026-09-17

Status

Accepted. Closes docs/gaps.md G41. Extends ADR-0081 and ADR-0228.

Context

§1.7’s frame loop is idle when nothing moves. A widget that draws from the clock keeps it turning through Paints.isAnimating(), which is how a spinner stays a spinner (ADR-0081). Canvas did not override it, so a canvas painted from CanvasStyle.nowMillis() was painted once. An application’s tile floor that settles, with each tile dropping in on a stagger, showed only its first frame.

An application cannot close this on its own. Implementing Paints itself means writing a second Canvas. A timer that calls host.repaint() repaints the whole window on a clock the frame pacer cannot see, so ADR-0271’s lateness measurement would count every one of those frames as late.

The question also cannot be a boolean on the description. A settle ends by itself when the last tile lands, and “has it landed” depends on the time. A boolean would make the application rebuild the widget to turn it off.

Decision

Canvas.animating(Predicate<CanvasStyle>), asked by the renderer once per frame, straight after render, with the CanvasStyle the painter was given.

• Paints gains isAnimating(ComputedStyle, Context), defaulting to the no-argument form. Every existing widget keeps its answer untouched, and the renderer calls only the new form.
• The renderer asks it inside the same window render runs in, before currentElement is cleared, so a context read by the answer is still this node’s. That is the rule ADR-0288 set for binding a painter, applied to a second reader.
• It is still asked after render, which is ADR-0228’s rule. The frame that draws the last tile landing answers false, and the loop goes quiet on the next frame rather than one frame later.
• The canvas snapshots context.canvasStyle(style) again for the predicate instead of keeping the painter’s copy. It is four values in a record, and keeping the copy would mean state on a record that has none.
• Reduced motion is the painter’s to honour. The predicate is handed reducedMotion(), and style -> !style.reducedMotion() is a loop that stops for a user who asked it to. The canvas does not guess, because a still drawing that needs no animation and one that loops are both legitimate for such a user.

Consequences

• Canvas is a four-component record. The three-component constructor stays, so every call site written before this compiles unchanged.
• new Canvas(floor::paint).animating(style -> style.nowMillis() - mounted < 2000) is the whole of an animated canvas. The showcase’s Motion screen is one (ADR-0354).
• Markup still names no painter, so animating has no markup form, for the reason the painter has none.