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docs: widget-leaflet extensions design
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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docs/plans/2026-03-06-widget-leaflet-extensions-design.md
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docs/plans/2026-03-06-widget-leaflet-extensions-design.md
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# Widget-Leaflet Extensions Design
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## Goal
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Extend the existing leaflet map widget adapter with bounding box drawing, image overlay, and marker support for the tessera geospatial classification workflow.
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## Architecture
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All changes are to the JS adapter code embedded in `widget_leaflet.ml`. No changes to the OCaml widget API — the existing `Widget.command` and handler system is sufficient. We add new command names and event types to the adapter.
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## New Commands
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### Bounding box drawing
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- `enableBboxDraw` — enables rectangle drawing mode via mousedown/mousemove/mouseup
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- User drags to draw a rectangle with visual feedback (dashed while dragging, solid green when complete)
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- Drawing a new rectangle replaces the previous one
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- New event `bbox_drawn` fires with `{south, west, north, east}` payload
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### Image overlay
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- `addImageOverlay` — takes `{url, bounds: [[south, west], [north, east]], opacity?}`. URL is typically a `data:image/png;base64,...` from tessera-viz-jsoo.
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- `removeImageOverlay` — removes the current overlay
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### Markers
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- `addMarker` — takes `{lat, lng, label?, color?}`. Adds a circle marker at the location.
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- `clearMarkers` — removes all markers
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## Usage from OCaml
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```ocaml
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(* Enable bbox drawing *)
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Widget.command ~id:"map" "enableBboxDraw" "";
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(* Handle bbox drawn event *)
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let on_bbox = function
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| Some json -> (* parse {south, west, north, east} *)
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| None -> ()
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in
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Widget.display_managed ~id:"map" ~kind:"leaflet-map" ~config ~handlers:[
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"click", on_click;
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"bbox_drawn", on_bbox;
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]
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(* Add image overlay *)
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Widget.command ~id:"map" "addImageOverlay"
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{|{"url":"data:image/png;base64,...","bounds":[[52.0,0.0],[52.1,0.2]]}|};
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(* Add training point markers *)
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Widget.command ~id:"map" "addMarker" {|{"lat":52.05,"lng":0.1,"color":"red"}|};
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```
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## Implementation approach
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Custom lightweight rectangle drawing using Leaflet's built-in `L.rectangle` + mouse events (~30 lines JS). No Leaflet.draw plugin dependency — avoids the toolbar UI issues the Python version fights against.
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## Testing
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Playwright test with a standalone HTML page that embeds the adapter JS directly, exercises commands, and verifies DOM state.