Fix custom layer names that are purely numeric being dropped

array_merge renumbers integer-like array keys, so a custom Leaflet layer
whose name is purely numeric (e.g. "1904") was silently dropped from the
available-layers whitelist and never rendered. Use the union operator
instead, which preserves such keys while keeping custom names taking
precedence over a same-named stock layer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JeroenDeDauw

src/LeafletService.php

@@ -298,10 +298,10 @@ private function filterToAvailable( array $values, array $available ): array {
 	 * @return array<string, bool>
 	 */
 	private function availableWithDefinitions( array $available ): array {
-		return array_merge(
-			$available,
-			array_fill_keys( $this->layerDefinitions->getLayerNames(), true )
-		);
+		// Union rather than array_merge: array_merge renumbers integer-like keys, which would drop
+		// a custom layer whose name is purely numeric (e.g. "1904"). Custom names go on the left so
+		// they take precedence over a same-named stock layer.
+		return array_fill_keys( $this->layerDefinitions->getLayerNames(), true ) + $available;
 	}
 
 	/**

tests/Unit/LeafletServiceTest.php

@@ -104,6 +104,20 @@ public function testCustomLayerDefinitionSurvivesFiltering() {
 		);
 	}
 
+	public function testNumericallyNamedCustomLayerSurvivesFiltering() {
+		$mapData = $this->newLeafletMapData(
+			[ 'layers' => [ 'OpenStreetMap', '1904' ] ],
+			new LeafletLayerDefinitions( [
+				'1904' => [ 'url' => 'https://tiles.example/{z}/{x}/{y}.png' ],
+			] )
+		);
+
+		$this->assertSame(
+			[ 'OpenStreetMap', '1904' ],
+			$mapData->getParameters()['layers']
+		);
+	}
+
 	public function testCustomOverlayDefinitionSurvivesFiltering() {
 		$mapData = $this->newLeafletMapData(
 			[ 'overlays' => [ 'OpenSeaMap', 'Historic', '<img src=x onerror="alert(1)">' ] ],