get-dateline-aware-extent.ts

const WGS84: ol.proj.ProjectionLike = 'EPSG:4326'
const WEB_MERCATOR: ol.proj.ProjectionLike = 'EPSG:3857'

function getDatelineAwareExtent(geometry: ol.geom.Geometry) {
  if (geometry instanceof ol.geom.MultiPolygon && crossesDateline(geometry)) {
    const extents = geometry.getPolygons().map(g => ol.proj.transformExtent(g.getExtent(), WEB_MERCATOR, WGS84))
    let [, minY, , maxY] = ol.proj.transformExtent(geometry.getExtent(), WEB_MERCATOR, WGS84)
    let width = 0
    let minX = 180
    for (const [polygonMinX, , polygonMaxX] of extents) {
      width += polygonMaxX - polygonMinX
      if (polygonMaxX > 0) {
        minX -= polygonMaxX - polygonMinX
      }
    }
    return ol.proj.transformExtent([minX, minY, minX + width, maxY], WGS84, WEB_MERCATOR)
  }
  return geometry.getExtent()  // Use as-is
}

function crossesDateline(geometry: ol.geom.Geometry) {
  const [minX, , maxX] = ol.proj.transformExtent(geometry.getExtent(), WEB_MERCATOR, WGS84)
  return minX === -180 && maxX === 180
}

test-fixture.geojson