What "ancillary" means here

Everything that is not the map. The map engines, layers, and tool registry live in Basics/ (core runtime). The user-facing tools live in Tools/. Everything else around the edges — the topbar search box, the coordinate readout under the cursor, the modal dialog system, the right-click menu, the scale bar, the login form, the landing page that lets a user pick a mission — lives in src/essence/Ancillary/ (with a small assist from Helpers/ and LandingPage/).

These modules are pre-React MMGIS. Most are plain ES modules built in a jQuery imperative style: a single object literal exporting init, remove, and a few helpers, with HTML constructed as a string and appended to a known DOM id.

Typical module shape

Open almost any file in Ancillary/ and you will see the same skeleton:

import $ from 'jquery'
import './ScaleBar.css'

var ScaleBar = {
    init: function (scaleBox) {
        Map_.map.on('zoomend', setScaleBars)
        Map_.map.on('moveend', setScaleBars)
        setScaleBars()
    },
    remove: function () { /* tear down listeners */ },
}

A few conventions that hold across the folder:

• Each *.js ships with a sibling *.css (Coordinates.js + Coordinates.css, Modal.js + Modal.css, etc.). The CSS is imported directly so Webpack bundles it.
• Markup is built as a prettier-ignore array of HTML strings joined with newlines, then appended via jQuery to a host element owned by the layout.
• State lives on the module object (e.g. Coordinates.mouseLngLat, Modal._activeModalIds) rather than in a framework store.
• Cross-module wiring is done by importing peers directly — ContextMenu reaches into Coordinates, Help uses Modal, almost everyone touches Basics/Layers_/Layers_ (the global L_), Basics/Map_/Map_, and Basics/Formulae_/Formulae_.

Where each piece is mounted

The module is defined under Ancillary/, but the DOM slot it lives in is created by the layout machinery in Basics/ — see core runtime for UserInterface_, ComponentController_, and PanelManager_. Booting in src/essence/essence.js is mostly a sequence of import then Thing.init(...) calls against those slots.

What lives in each subfolder

Ancillary/ itself is a flat bag of single-purpose modules. Roughly grouped:

• Topbar / persistent chrome: Search.js (autocomplete + geodataset search), Coordinates.js (cursor lng/lat readout with switchable projections), Login/Login.js (sign-in / sign-up modal + session UI), MapLogo.js, Attributions.js, Help.js (markdown-rendered help modals via showdown).
• Map overlays: ScaleBar.js and ScaleBox.js (D3-driven scale on the Leaflet map), Compass.js, CursorInfo.js (the floating tooltip that follows the mouse), ContextMenu.js (the map right-click menu), Description.js.
• System utilities used as UI: Modal.js and ConfirmationModal.js (the generic modal stack everything else builds on), QueryURL.js (read/write the large set of ?mapLat=…&on=…&tools=… URL params that make MMGIS shareable), Stylize.js, LocalFilterer.js, DataShaders.js, Sprites.js (THREE.js sprite factory used by the 3D globe), Swap.js.

Helpers/ is nearly empty in the JS tree — just a stray PHP utility for DEM tile elevation lookup. Treat it as legacy.

LandingPage/ is one module, LandingPage.js, that runs before the map boots: it inspects ?mission= via QueryURL, falls back to MAIN_MISSION, and either hands control to the chosen mission config or renders a mission-picker grid.