Fix conversion of max zoom level from mapbox styles

The QGIS renderers for vector tiles treat maxzoom
different to the MapBox Style Specifications.

From the MapBox Specifications:

"The maximum zoom level for the layer. At zoom levels
equal to or greater than the maxzoom, the layer will be hidden."

However the QGIS styles will be hidden if the zoom
level is GREATER THAN (not equal to) maxzoom.

Accordingly we need to subtract 1 from the maxzoom value when
converting mapbox styles

src/core/vectortile/qgsmapboxglstyleconverter.cpp

@@ -136,7 +136,17 @@ void QgsMapBoxGlStyleConverter::parseLayers( const QVariantList &layers, QgsMapB
     const QString layerName = jsonLayer.value( QStringLiteral( "source-layer" ) ).toString();
 
     const int minZoom = jsonLayer.value( QStringLiteral( "minzoom" ), QStringLiteral( "-1" ) ).toInt();
-    const int maxZoom = jsonLayer.value( QStringLiteral( "maxzoom" ), QStringLiteral( "-1" ) ).toInt();
+
+    // WARNING -- the QGIS renderers for vector tiles treat maxzoom different to the MapBox Style Specifications.
+    // from the MapBox Specifications:
+    //
+    // "The maximum zoom level for the layer. At zoom levels equal to or greater than the maxzoom, the layer will be hidden."
+    //
+    // However the QGIS styles will be hidden if the zoom level is GREATER THAN (not equal to) maxzoom.
+    // Accordingly we need to subtract 1 from the maxzoom value in the JSON:
+    int maxZoom = jsonLayer.value( QStringLiteral( "maxzoom" ), QStringLiteral( "-1" ) ).toInt();
+    if ( maxZoom != -1 )
+      maxZoom--;
 
     const bool enabled = jsonLayer.value( QStringLiteral( "visibility" ) ).toString() != QLatin1String( "none" );
 

tests/src/python/test_qgsmapboxglconverter.py

@@ -941,6 +941,72 @@ def testLabelWithField(self):
         self.assertFalse(labeling_style.labelSettings().isExpression)
         self.assertEqual(labeling_style.labelSettings().fieldName, 'substance')
 
+    def test_parse_zoom_levels(self):
+        context = QgsMapBoxGlStyleConversionContext()
+        style = {
+            "sources": {
+                "Basemaps": {
+                    "type": "vector",
+                    "url": "https://xxxxxx"
+                }
+            },
+            "layers": [
+                {
+                    "id": "water",
+                    "source": "streets",
+                    "source-layer": "water",
+                    "minzoom": 3,
+                    "maxzoom": 11,
+                    "type": "fill",
+                    "paint": {
+                        "fill-color": "#00ffff"
+                    }
+                },
+                {
+                    "layout": {
+                        "text-field": "{name_en}",
+                        "text-font": [
+                            "Open Sans Semibold",
+                            "Arial Unicode MS Bold"
+                        ],
+                        "text-max-width": 8,
+                        "text-anchor": "top",
+                        "text-size": 11,
+                        "icon-size": 1
+                    },
+                    "type": "symbol",
+                    "id": "poi_label",
+                    "minzoom": 3,
+                    "maxzoom": 11,
+                    "paint": {
+                        "text-color": "#666",
+                        "text-halo-width": 1.5,
+                        "text-halo-color": "rgba(255,255,255,0.95)",
+                        "text-halo-blur": 1
+                    },
+                    "source-layer": "poi_label"
+                }
+            ]
+        }
+
+        converter = QgsMapBoxGlStyleConverter()
+        converter.convert(style, context)
+
+        renderer = converter.renderer()
+        style = renderer.style(0)
+        self.assertEqual(style.minZoomLevel(), 3)
+        # This differs from the handling of the max zoom as defined
+        # in the MapBox Style, since in MapBox styles the style is rendered
+        # only if the zoom level is less than the maximum zoom but in QGIS
+        # styles the style is rendered if the zoom level is less than OR EQUAL TO
+        # the maximum zoom
+        self.assertEqual(style.maxZoomLevel(), 10)
+
+        labeling = converter.labeling()
+        style = labeling.style(0)
+        self.assertEqual(style.minZoomLevel(), 3)
+        self.assertEqual(style.maxZoomLevel(), 10)
+
     def test_parse_raster_source(self):
         context = QgsMapBoxGlStyleConversionContext()
         style = {