Add GeoJSON layer overlay type replacing KmlLayer

[rabauss]

Google deprecated the KmlLayer class of the Maps JavaScript API. It becomes unavailable in version 3.66 (August 2026) and stops working entirely in May 2027.

This adds a GeoJSON layer overlay type built on google.maps.Data as the replacement path, plus a command to convert existing KML files. The KML type is left in place so existing maps keep rendering until they are migrated.

What's in it

• new overlay type with DCA fields, subpalettes for file/URL source, and DE/EN labels
• prepareGeoJsonLayer() using Ivory's existing GeoJsonLayer — the library needs no changes
• frontend script applying styling and info windows
• huh:google-maps:convert-kml command plus a reusable KmlConverter

Why the frontend script is needed

GeoJsonLayer renders map.data.loadGeoJson() and nothing else. Unlike KML, GeoJSON carries no styling of its own, so a layer loaded that way draws in Google's default blue with no info windows.

The script applies both: styling via Data.setStyle(), reading each feature's simplestyle properties with the overlay's configured style as fallback, and info windows built from the remaining properties. Its markup uses BEM classes and ships no presentation of its own, so projects style it themselves.

Two non-obvious bits

Both look like they could be simplified, and both break silently if they are:

• The initialization is registered as a map event. Ivory renders events inside the map's own init callback; the rendered JavaScript block runs before the async API callback assigns the map variable.
• The script is registered from the replaceDynamicScriptTags hook. Maps render inside the body, by which point the head has been assembled — a TL_JAVASCRIPT entry added during rendering is dropped without warning.

Conversion

The common KML converters drop the styling, since GeoJSON has no place to put <Style> definitions and every route ends up the same colour. KmlConverter flattens them into simplestyle properties instead, resolves <StyleMap> indirections via their normal branch, converts KML's AABBGGRR colours (alpha first, then reversed RGB — read naively, red and blue swap), closes linear rings as GeoJSON requires, and keeps ExtendedData. Icon URLs can optionally be rewritten to local paths.

Usage is documented in the README.

Notes

• Based on feature/contao-5.7 (Unlock symfony 7 heimrichhannot/contao-google-maps-bundle#37), not v3 — it builds on the Contao 5.7 work.
• No unit tests yet. kmlColorToHex() and the StyleMap resolution would be the ones worth covering; happy to add them if you want this merged.

[rabauss]

Superseded by heimrichhannot#38, which targets the upstream repository.