A child pinned with bottom: 0 can end a pixel above its parent's bottom after pixel rounding

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Dal 6/11/2026.

  • #2068 di @Pduhard — aperta

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
25/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Ferma
Stack tecnologico
cpp, react-native
Ambito
frontend, testing

Direzione di ricerca

Start with PixelGrid.cpp and the roundLayoutResultsToPixelGrid logic described in the issue. Review draft PR #2068 and the gentest position rounding and generated expectations it identifies. Done means bottom-pinned children align with the parent after rounding, text positions avoid cumulative drift, and the reported gentest expectations are updated.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Summary

  • roundLayoutResultsToPixelGrid: position rounded from relative offset (nodeTop), size from absolute edges (round(absoluteNodeBottom) - round(absoluteNodeTop)).
  • Parent on a fractional pixel → child pinned bottom: 0 ends 1 px above parent's bottom.

Repro

yoga-layout 3.2.1; PixelGrid.cpp on main rounds the same way.

import Yoga, { Edge, PositionType } from "yoga-layout";

const config = Yoga.Config.create();
config.setPointScaleFactor(1);

const root = Yoga.Node.create(config);
root.setWidth(100);

const parent = Yoga.Node.create(config);
parent.setPositionType(PositionType.Absolute);
parent.setPosition(Edge.Top, 0.25);
parent.setWidth(100);
parent.setHeight(10.5);

const child = Yoga.Node.create(config);
child.setPositionType(PositionType.Absolute);
child.setPosition(Edge.Bottom, 0);
child.setWidth(100);
child.setHeight(10.25);

parent.insertChild(child, 0);
root.insertChild(parent, 0);
root.calculateLayout(undefined, undefined);

parent.getComputedLayout(); // { top: 0, height: 11 }
child.getComputedLayout();  // { top: 0, height: 10 } -> bottom 10, parent bottom 11
  • Expected: child top: 1, height: 10, flush with parent bottom.
  • Actual: 1 px gap.

Why

  • Child absolute edges 0.5 / 10.75 → rounded 1 / 11 → height 10.
  • Position: round(0.25) = 0, not round(0.5) - round(0.25) = 1.

Scale

  • Scale 2.25 (Pixel 8 Pro), parent 506 tall at offset 0–4, bottom: 0 child 150–260 tall: 2003 / 9176 layouts 1 px short.
  • top: 0; bottom: 0 child: 0 / 9176.
  • Seen in React Native 0.88, Android: card shade bottom: 0 stops 1 px above the card edge, image line shows.

Related

  • #1574: rounding is meant to avoid gaps between touching edges. This is one.

Possible fix

  • Non-text position: round(absoluteNodeTop) - round(absoluteTop) (same for left), reusing the rounded leading edge of the size.
  • Text nodes keep floored relative position: absolute flooring drifts text 1 px per relayout.
  • gentest reads positions with the same relative rounding (Math.round(rect.top - parentTop)); change it too. 2 generated expectations move.
  • Draft PR: #2068.

Written with help from Claude (AI); reviewed and tested by me.

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