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.

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.

[nina-mir]

Happy to explain!

The chain begins with the L.geoJson(...) call we emit, which is populated through addData. That method runs once for each feature and performs three steps in sequence: build a layer with geometryToLayer, assign feature to it, then call onEachFeature (GeoJSON.js:118-129, v1.9.3).

The catch is what geometryToLayer returns. For most geometries it's a single layer, but for MultiPoint and GeometryCollection it's a FeatureGroup built from child layers. And, addData assigns feature to whatever it got back. So for a MultiPoint the group gets feature and the markers inside it get nothing, which is a problem for tooltip binding!

GeoJsonTooltip binds one tooltip on the whole GeoJSON layer, with a function as its content. When the tooltip opens, Leaflet picks the layer to hand that function via this._tooltip._source = e.layer || e.target (Tooltip.js:418, v1.9.3) — the layer that fired the event, i.e. the child marker under the cursor. That marker has no feature, so layer.feature.properties[...] throws before anything renders.

So the two ends disagree: feature is assigned to the group, the content function is handed a child. Instrumenting onEachFeature shows it directly — for a MultiPoint, layer.getLayers().map(c => c.feature !== undefined) is [false] before the patch and [true] after.

The patch copies the feature down onto the children inside onEachFeature, which is the one point where both the feature data and the constructed layer are accessible. The recursion is for a GeometryCollection containing a MultiPoint, where the marker sits two levels below the layer that owns feature.

It also explains why MultiLineString and MultiPolygon were never affected — Leaflet flattens those into a single Polyline/Polygon, so the layer you hover is the layer that owns feature.

On upstream: I tested it outside folium via plain Leaflet from a CDN, the reporter's coordinates, tooltip content as a function reading layer.feature. Same behavior in 1.9.3 (our pin), 1.9.4, and 2.0.0-alpha.1: Point works, MultiPoint throws. So it isn't something folium introduces, but it also looks deliberate rather than accidental —
addData assigns feature to the layer geometryToLayer returned, and _openTooltip
resolves to the layer that fired the event. Both rules are reasonable alone; they disagree when the geometry produces a group. 2.x renamed e.layer to e.propagatedFrom and kept the same resolution, so the rewrite didn't change it.

I couldn't find an existing Leaflet issue for this. Happy to open one with the repro if you think it's worth raising — though with 1.x in maintenance and 2.x behaving the same, handling it here seems worthwhile either way. Glad to share the test pages too.

[nina-mir]

Test added. It fails on main (tooltip never renders, wait_until times out) and passes here.

Two notes on how it's written:

• It's the first hover-based test in tests/selenium/, and the hover is dispatched as a DOM MouseEvent rather than through ActionChains — Selenium's pointer actions don't reliably reach Leaflet's SVG paths in headless Chrome, while a bubbling event goes through Map._handleDOMEvent the same way a real hover does.

• It deliberately omits verify_js_logs(): Leaflet 1.9.3 throws t.getElement is not a function from _addFocusListenersOnLayer for any GeoJSON layer containing a nested FeatureGroup, regardless of this fix. That's guarded upstream in 1.9.4, so a version bump would let the assertion come back. Happy to open that separately.

[hansthen]

I will have a look at it tomorrow. Sorry for the delay in responding. I have had guests over and not much time for life online.

[nina-mir]

thank you! no rush on my part. I wish you a wonderful week! :-)

[hansthen]

Hi @nina-mir I merged the PR but there is still an issue of the failing Selenium tests. Do you think these will be solved by the version bump you mentioned? Are you willing to create a separate PR for that?

[nina-mir]

Hi @hansthen, Thank you! Yes, the error will go away with the version bump. I am on it. Will do the PR right away!