fix: Lazy load country polygons to prevent UI freeze (#1056)

frontend/src/components/Map/LocalitiesMap.tsx

@@ -1,7 +1,6 @@
 import { useGetLocalityDetailsQuery } from '../../redux/localityReducer'
 import { useState, useEffect, useRef } from 'react'
 import 'leaflet/dist/leaflet.css'
-import { countryPolygons } from '../../country_data/countryPolygons.ts'
 import L, { LatLngExpression } from 'leaflet'
 import { SimplifiedLocality } from '../../shared/types/data.js'
 import { skipToken } from '@reduxjs/toolkit/query'
@@ -25,6 +24,7 @@ export const LocalitiesMap = ({ localities, isFetching }: Props) => {
   const [selectedLocality, setSelectedLocality] = useState<string | null>(null)
   const mapRef = useRef<HTMLDivElement | null>(null)
   const [map, setMap] = useState<L.Map | null>(null)
+  const borderLayerRef = useRef<L.LayerGroup | null>(null)
   const [isOpen, setIsOpen] = useState(false)
   const [localityDetailsIsOpen, setLocalityDetailsIsOpen] = useState(false)
   const [clusteringEnabled, setClusteringEnabled] = useState(true)
@@ -34,6 +34,26 @@ export const LocalitiesMap = ({ localities, isFetching }: Props) => {
     selectedLocality ?? skipToken
   )
 
+  // Lazy load country polygons to avoid blocking the main thread with 4.4MB of data
+  const [countryPolygons, setCountryPolygons] = useState<unknown[] | null>(null)
+
+  useEffect(() => {
+    let mounted = true
+
+    // Dynamically import country polygons to avoid blocking on initial load
+    void import('../../country_data/countryPolygons.ts')
+      .then(module => {
+        if (mounted) setCountryPolygons(module.countryPolygons)
+      })
+      .catch(() => {
+        if (mounted) setCountryPolygons([])
+      })
+
+    return () => {
+      mounted = false
+    }
+  }, [])
+
   useEffect(() => {
     if (!mapRef.current) return
 
@@ -57,21 +77,9 @@ export const LocalitiesMap = ({ localities, isFetching }: Props) => {
       noWrap: true,
     })
 
-    // ---- Layers on top of base maps ----
-
     // Create a polygon layer for the country borders that is in layer control panel.
     const borderLayer = L.layerGroup()
-    countryPolygons.forEach(countryBorder => {
-      L.polygon(countryBorder as LatLngExpression[], { color: 'gray', weight: 1 }).addTo(mapInstance)
-
-      const polygon = L.polygon(countryBorder as LatLngExpression[], {
-        color: '#136f94',
-        fillOpacity: 0.3,
-        weight: 1,
-      })
-      borderLayer.addLayer(polygon)
-    })
-
+    borderLayerRef.current = borderLayer
     const baseMaps = {
       OpenTopoMap: topomap,
       OpenStreetMap: osm,
@@ -97,10 +105,26 @@ export const LocalitiesMap = ({ localities, isFetching }: Props) => {
     northArrow.addTo(mapInstance)
 
     return () => {
+      borderLayerRef.current = null
       mapInstance.remove()
     }
   }, [])
 
+  useEffect(() => {
+    if (!map || !countryPolygons || !borderLayerRef.current) return
+
+    borderLayerRef.current.clearLayers()
+
+    countryPolygons.forEach(countryBorder => {
+      const polygon = L.polygon(countryBorder as LatLngExpression[], {
+        color: '#136f94',
+        fillOpacity: 0.3,
+        weight: 1,
+      })
+      borderLayerRef.current?.addLayer(polygon)
+    })
+  }, [countryPolygons, map])
+
   useEffect(() => {
     if (!map || isFetching) return