Use CSS z-index to order layers

rendering/cases/vector-zindex/main.js

@@ -0,0 +1,50 @@
+import Map from '../../../src/ol/Map.js';
+import View from '../../../src/ol/View.js';
+import Feature from '../../../src/ol/Feature.js';
+import LineString from '../../../src/ol/geom/LineString.js';
+import VectorLayer from '../../../src/ol/layer/Vector.js';
+import VectorSource from '../../../src/ol/source/Vector.js';
+import Style from '../../../src/ol/style/Style.js';
+import Stroke from '../../../src/ol/style/Stroke.js';
+
+
+const vectorSourceRed = new VectorSource();
+const vectorSourceBlue = new VectorSource();
+let feature;
+
+feature = new Feature({
+  geometry: new LineString([[-60, 20], [45, 20]])
+});
+feature.setStyle(new Style({
+  stroke: new Stroke({color: '#f00', width: 10})
+}));
+vectorSourceRed.addFeature(feature);
+
+
+feature = new Feature({
+  geometry: new LineString([[0, -50], [0, 60]])
+});
+feature.setStyle(new Style({
+  stroke: new Stroke({color: '#00f', width: 16})
+}));
+vectorSourceBlue.addFeature(feature);
+
+new Map({
+  pixelRatio: 1,
+  layers: [
+    new VectorLayer({
+      zIndex: 1,
+      source: vectorSourceRed
+    }),
+    new VectorLayer({
+      source: vectorSourceBlue
+    })
+  ],
+  target: 'map',
+  view: new View({
+    center: [0, 0],
+    resolution: 1
+  })
+});
+
+render();

src/ol/renderer/Composite.js

@@ -1,12 +1,11 @@
 /**
  * @module ol/renderer/canvas/Map
  */
-import {stableSort} from '../array.js';
 import {CLASS_UNSELECTABLE} from '../css.js';
 import {visibleAtResolution} from '../layer/Layer.js';
 import RenderEvent from '../render/Event.js';
 import RenderEventType from '../render/EventType.js';
-import MapRenderer, {sortByZIndex} from './Map.js';
+import MapRenderer from './Map.js';
 import SourceState from '../source/State.js';
 import {replaceChildren} from '../dom.js';
 
@@ -79,8 +78,6 @@ class CompositeMapRenderer extends MapRenderer {
     this.dispatchRenderEvent(RenderEventType.PRECOMPOSE, frameState);
 
     const layerStatesArray = frameState.layerStatesArray;
-    stableSort(layerStatesArray, sortByZIndex);
-
     const viewResolution = frameState.viewState.resolution;
 
     this.children_.length = 0;
@@ -91,7 +88,14 @@ class CompositeMapRenderer extends MapRenderer {
       }
 
       const layer = layerState.layer;
-      this.children_.push(layer.render(frameState));
+      const element = layer.render(frameState);
+      if (element) {
+        const zIndex = layerState.zIndex;
+        if (zIndex !== element.style.zIndex) {
+          element.style.zIndex = zIndex;
+        }
+        this.children_.push(element);
+      }
     }
 
     replaceChildren(this.element_, this.children_);