Issue 1520: Propagate feature to child layers of multi-geometries

[nina-mir]

Leaflet's geometryToLayer returns a FeatureGroup for MultiPoint, and addData assigns feature to that group only. Tooltips resolve their source to the layer that fired the event, which is a child marker with no feature, so the content function throws before rendering. This copies the feature onto child layers in onEachFeature, descending recursively so that nested groups — a GeometryCollection containing a MultiPoint — are also covered.

Draft [still to come]: a selenium regression test.

Verified in Firefox/Chrome against the reporter's snippet and a matrix of all eight geometry types. MultiLineString and MultiPolygon are unaffected, since Leaflet flattens those into a single layer rather than a group.

Geometry	Leaflet returns	Before	After
Point, LineString, Polygon	single layer	works	works
MultiLineString, MultiPolygon	single layer (flattened)	works	works
MultiPoint	FeatureGroup	broken	works
GeometryCollection	FeatureGroup	broken	works
GeometryCollection containing MultiPoint	nested FeatureGroup	broken	works

Console error before the fix

Uncaught TypeError: can't access property "properties", layer.feature is undefined
    <anonymous> 
    _updateContent DivOverlay.js:277
    update DivOverlay.js:187
    onAdd DivOverlay.js:113
    onAdd Tooltip.js:84
    _layerAdd Layer.js:114
    whenReady Map.js:1477
    addLayer Layer.js:172
    openOn DivOverlay.js:63
    openTooltip Tooltip.js:337
    _openTooltip Tooltip.js:420
    fire Events.js:195
    _propagateEvent Events.js:311
    fire Events.js:204
    _propagateEvent Events.js:311
    fire Events.js:204
    _propagateEvent Events.js:311
    fire Events.js:204
    _fireDOMEvent Map.js:1452
    _handleDOMEvent Map.js:1401
    o DomEvent.js:108

If this approach looks right, the MultiPoint limitation in the GeoJSON docs and the GeometryCollection warning in GeoJsonDetail would need updating. Happy to do that here or in a follow-up.

Tested with

• folium 0.20.0 (reproduction) and this branch off 03cb432
• Leaflet 1.9.3, as pinned in folium/folium.py
• Firefox 153.0.1 and Chrome 151 on Linux

Two tests fail on this branch, both of which also fail on clean main:

• tests/test_map.py::test_icon_invalid_marker_colors — Icon.__init__ calls color.startswith("#") before validating the type, so color=42 raises AttributeError instead of emitting the expected UserWarning (folium/map.py:436). Unrelated to this change; happy to open a separate issue or PR.

• tests/plugins/test_time_slider_choropleth.py::test_timedynamic_geo_json — geodatasets not installed locally.

tests/test_features.py: 20 passed. tests/test_map.py and tests/plugins: 91 passed, 2 failed (both above).

Fixes #1520

[hansthen]

@nina-mir This looks like a promising and elegant approach. I must admit I do not grasp quite how this change solves the issue. Could you explain this a bit more (just for my curiosity)?

Interested to see the regression tests. I'd be happy to accept this here. Isn't this also an upstream problem? It looks like an issue in Leaflet itself as well.