Adjust center of world copy in which markers are drawn to match default center of map

Here's a way to do this currently — not the most robust since it messes with Leaflet's internal JavaScript functions:

(function () {

  // The Leaflet module doesn't have a JavaScript API, only a PHP API. But it would be nice to be able
  // to use constants and functions defined in the Leaflet library rather than reimplementing them in PHP.
  // So we use the same trick that the Leaflet More Maps module uses to override Leaflet's behavior.

  Drupal.Leaflet.prototype._initialise_orig = Drupal.Leaflet.prototype.initialise;

  Drupal.Leaflet.prototype.initialise = function(mapid) {
    Drupal.Leaflet.prototype._initialise_orig.call(this, mapid);

    if (this.map_settings.center) {
      const minLng = this.map_settings.center.lon - 180;
      const maxLng = this.map_settings.center.lon + 180;

      // Recenter the map wrapping so that the copy of the world in which markers will be placed aligns
      // with the initial viewport.
      this.lMap.crs = L.CRS.EPSG3857;
      this.lMap.crs.wrapLng = [minLng, maxLng];
    }
  };

  Drupal.Leaflet.prototype._create_point_orig = Drupal.Leaflet.prototype.create_point;

  Drupal.Leaflet.prototype.create_point = function(marker) {
    const lMarker = Drupal.Leaflet.prototype._create_point_orig.call(this, marker);

    // Place markers within the copy of the world that aligns with the initial viewport.
    let latLng = lMarker.getLatLng();
    latLng = this.lMap.wrapLatLng(latLng);
    lMarker.setLatLng(latLng);
    return lMarker;
  }

})();

@jlstrecker doesn't this comment of you (and related Fixed issue) solve this issue too?

Please close and mark this as Fixed in that case.

The code in comment #2 did solve my problem, at least for now. I am concerned that it may break with future updates to the Leaflet module, since it messes with Leaflet's internal functions. Which is why I had proposed providing an option within Views or hook_leaflet_map_info_alter(). However, if you feel that it's a stable enough solution, and don't see a need to make the option more easily accessible, then I am OK with closing the issue.

sorry sorry my bad ... @jlstrecker
I wanted to point to this comment of you (and that related issue):
https://www.drupal.org/project/leaflet/issues/3129345#comment-15815947 💬 Displaying marks as map pans Active

Doesn't it also solve also this one for you?

Oh, I see what you mean. Actually, worldCopyJump does not solve the problem.

The reason that worldCopyJump doesn't solve the problem is that behaves a bit surprisingly, both when panning and when showing the map initially. As an example, let's say you have one marker in the US, one in France, and one in China. Once you pan far enough to the right or left, all three markers jump at the same time. It's a bit weird — I would have expected them to jump one at a time — but that's how it works.

When you have the map configured with non-default center and zoom as described in this issue's description, and additionally have worldCopyJump enabled, what happens when you show the map initially? The markers for the US and France are visible, but the marker for China is in the copy of China that is offscreen to the right, not the one that is showing on the map. Only after panning to the left will the marker appear in the onscreen copy of China.

I actually ended up not using worldCopyJump for the project I was working on, but instead added a line to the JS code above to prevent the user from panning to a copy of the world that lacks markers:

this.lMap.setMaxBounds(L.latLngBounds(L.latLng(-90, minLng), L.latLng(90, maxLng)));

thanks @jlstrecker for better describing the use case you wanted to solve ..
and Only now I had the occasion (let me confess) to have a look to your custom code implementation from #2.
Smart one ... basically you are extending / overriding the existing Drupal.Leaflet.prototype.initialise and Drupal.Leaflet.prototype.create_point methods ...
.
BUT (? question) wouldn't you be able to accomplish the same results taking advantage of the 'leafletMapInit' event (or the same 'leaflet.map' one), defined here:
https://git.drupalcode.org/project/leaflet/-/blob/10.2.x/js/leaflet.drup...
and available/triggered the first time the Leaflet Map is loaded, and then interact with (or even alter) the Leaflet Map itself (data.lMap) and its markers (data.markers) adding your code (and new Leaflet Popup plugin or whatever else) the way you prefer ...

This long standing issue was created to better explain all that (how to interact and alter with generated Drupal Leaflet Maps and its Properties, Features etc.)

That isse also drives you on how to better inspect that Live Demo Drupal Leaflet additional Drupal.behaviors.geofieldMapLeafletMapInteraction ....

Let me know if this could also help you, or Not, you still need to alter the original methods.

It's been a while since I worked on it, and I don't remember why I didn't use the leafletMapInit event — if I tried it and ran into some problem, or if I just couldn't figure out how to use it so copied the approach from Leaflet More Maps.

At some point I may revisit that code and see if I can rewrite it as you suggest. Until then, feel free to mark this issue Postponed or Closed.

This long standing issue was created to better explain all that (how to interact and alter with generated Drupal Leaflet Maps and its Properties, Features etc.)

That sounds helpful. Which issue are you referring to?

Ah super sorry, I missed the issue I was referring to.
Here it is:
https://www.drupal.org/project/leaflet/issues/3047091 📌 Leaflet Map & Markers external interaction, on events - LIVE DEMO Needs review

Automatically closed - issue fixed for 2 weeks with no activity.