Display multiple maps that share the same layer.

100% Map style by Stamen Design, © OpenStreetMap contributors
index.js 
import { MapControls } from "three/examples/jsm/controls/MapControls.js";

import Extent from "@giro3d/giro3d/core/geographic/Extent.js";
import Instance from "@giro3d/giro3d/core/Instance.js";
import ColorLayer from "@giro3d/giro3d/core/layer/ColorLayer.js";
import Map from "@giro3d/giro3d/entities/Map.js";
import Inspector from "@giro3d/giro3d/gui/Inspector.js";
import StadiaMaps from "ol/source/StadiaMaps.js";
import TiledImageSource from "@giro3d/giro3d/sources/TiledImageSource.js";

const extent = new Extent(
  "EPSG:3857",
  -20037508.342789244,
  20037508.342789244,
  -20037508.342789244,
  20037508.342789244,
);

const instance = new Instance({
  target: "view",
  crs: extent.crs,
  backgroundColor: 0x0a3b59,
});

// Adds an TMS imagery layer
const layer = new ColorLayer({
  name: "osm",
  source: new TiledImageSource({
    source: new StadiaMaps({ layer: "stamen_watercolor", wrapX: false }),
  }),
});

let index = 0;
const promises = [];
for (const ex of extent.split(8, 8)) {
  const mapExtent = ex.withRelativeMargin(-0.05);
  // Creates a map that will contain the layer
  const map = new Map({ extent: mapExtent });
  map.name = `${index++}`;

  instance.add(map);

  const promise = map.addLayer(layer).catch((e) => console.error(e));
  promises.push(promise);
}

Promise.allSettled(promises).then(() => {
  instance.view.camera.position.set(0, 0, 25000000);

  const controls = new MapControls(instance.view.camera, instance.domElement);
  instance.view.setControls(controls);

  Inspector.attach("inspector", instance);
});
index.html 
<!doctype html>
<html lang="en">
  <head>
    <title>Share layer between maps</title>
    <meta charset="UTF-8" />
    <meta name="name" content="share_layers_between_maps" />
    <meta
      name="description"
      content="Display multiple maps that share the same layer."
    />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />

    <link rel="icon" href="https://giro3d.org/images/favicon.svg" />
    <link
      href="https://giro3d.org/assets/bootstrap-custom.css"
      rel="stylesheet"
    />
    <script src="https://giro3d.org/assets/bootstrap.bundle.min.js"></script>
    <link
      rel="stylesheet"
      type="text/css"
      href="https://giro3d.org/next/examples/css/example.css"
    />
  </head>

  <body>
    <div id="view" class="m-0 p-0 w-100 h-100"></div>
    <div
      id="inspector"
      class="position-absolute top-0 start-0 mh-100 overflow-auto"
    ></div>

    <script type="module" src="index.js"></script>
    <script>
      /* activate popovers */
      const popoverTriggerList = [].slice.call(
        document.querySelectorAll('[data-bs-toggle="popover"]'),
      );
      popoverTriggerList.map(
        // bootstrap is used as script in the template, disable warning about undef
        // eslint-disable-next-line no-undef
        (popoverTriggerEl) =>
          new bootstrap.Popover(popoverTriggerEl, {
            trigger: "hover",
            placement: "left",
            content: document.getElementById(
              popoverTriggerEl.getAttribute("data-bs-content"),
            ).innerHTML,
            html: true,
          }),
      );
    </script>
  </body>
</html>
package.json 
{
    "name": "share_layers_between_maps",
    "dependencies": {
        "@giro3d/giro3d": "git+https://gitlab.com/giro3d/giro3d.git"
    },
    "devDependencies": {
        "vite": "^3.2.3"
    },
    "scripts": {
        "start": "vite",
        "build": "vite build"
    }
}