fix(core): notify layer observer when source-layer or source-id changes (#4372)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mbgl/style/layer.cpp

@@ -54,6 +54,7 @@ void Layer::setSourceLayer(const std::string& sourceLayer) {
     auto impl_ = mutableBaseImpl();
     impl_->sourceLayer = sourceLayer;
     baseImpl = std::move(impl_);
+    observer->onLayerChanged(*this);
 }
 
 void Layer::setSourceID(const std::string& sourceID) {
@@ -61,6 +62,7 @@ void Layer::setSourceID(const std::string& sourceID) {
     auto impl_ = mutableBaseImpl();
     impl_->source = sourceID;
     baseImpl = std::move(impl_);
+    observer->onLayerChanged(*this);
 };
 
 const Filter& Layer::getFilter() const {

test/style/style_layer.test.cpp

@@ -285,6 +285,44 @@ TEST(Layer, Observer) {
     };
     layer->setLineCap(lineCap);
     EXPECT_FALSE(layoutPropertyChanged);
+
+    // Notifies observer on source-layer change.
+    // This is required so that the render orchestrator re-evaluates already-loaded
+    // tiles when a layer's source-layer is changed at runtime (e.g. after addLayer).
+    bool sourceLayerChanged = false;
+    observer.layerChanged = [&](Layer& layer_) {
+        EXPECT_EQ(layer.get(), &layer_);
+        sourceLayerChanged = true;
+    };
+    layer->setSourceLayer("roads");
+    EXPECT_TRUE(sourceLayerChanged);
+
+    // Does not notify observer on no-op source-layer change.
+    sourceLayerChanged = false;
+    observer.layerChanged = [&](Layer& layer_) {
+        EXPECT_EQ(layer.get(), &layer_);
+        sourceLayerChanged = true;
+    };
+    layer->setSourceLayer("roads");
+    EXPECT_FALSE(sourceLayerChanged);
+
+    // Notifies observer on source ID change.
+    bool sourceIDChanged = false;
+    observer.layerChanged = [&](Layer& layer_) {
+        EXPECT_EQ(layer.get(), &layer_);
+        sourceIDChanged = true;
+    };
+    layer->setSourceID("other-source");
+    EXPECT_TRUE(sourceIDChanged);
+
+    // Does not notify observer on no-op source ID change.
+    sourceIDChanged = false;
+    observer.layerChanged = [&](Layer& layer_) {
+        EXPECT_EQ(layer.get(), &layer_);
+        sourceIDChanged = true;
+    };
+    layer->setSourceID("other-source");
+    EXPECT_FALSE(sourceIDChanged);
 }
 
 TEST(Layer, DuplicateLayer) {