Fill a tldraw shape with lines.

fillShapeWithLines.ts

function fillRectangleWithLines(
  shape: Rectangle,
  gap: number,
  messiness = 0.5
) {
  const { size } = shape

  const rng = Utils.rng(shape.shape.id)

  const gapX = size[0] / (Math.floor((size[0] - gap) / gap) / 2)
  const gapY = size[1] / (Math.floor((size[1] - gap) / gap) / 2)

  let left = [-gap / 2, -gap / 2]
  let right = [-gap / 2, -gap / 2]

  const points: number[][] = []

  while (true) {
    points.push(
      ...Utils.getPointsBetween(
        Vec.add(right, [rng() * gap * messiness, rng() * gap * messiness]),
        left,
        {
          steps: 3,
          ease: (t) => t * t * t,
        }
      )
    )

    if (left[0] >= size[0]) {
      left[1] += gapY
    } else {
      left[0] += gapX
    }

    points.push(
      ...Utils.getPointsBetween(
        Vec.add(left, [rng() * gap * messiness, rng() * gap * messiness]),
        right,
        {
          steps: 3,
          ease: (t) => t * t * t,
        }
      )
    )

    if (right[1] >= size[1]) {
      right[0] += gapX
    } else {
      right[1] += gapY
    }

    if (right[0] >= size[0] - gap || left[1] >= size[1] - gap) {
      break
    }
  }

  return new Draw({
    point: shape.point,
    points,
    style: {
      size: SizeStyle.Large,
      color: ColorStyle.Blue,
    },
    childIndex: shape.childIndex - 1,
  })
}

Sorta demo: https://codesandbox.io/s/sketch-rect-lb0r9