Stop single-layer mode from showing the layer below (#482)

startLayer is inclusive: the bottom clipping plane sits at the bottom of
layer startLayer. Collapsing the range to endLayer - 1 therefore left two
layers visible. Collapse it onto endLayer itself, in both the endLayer
setter and the singleLayerMode setter.

Fixes #450

Author: sophiedeziel

src/__tests__/scene-manager-properties.ts

@@ -774,7 +774,18 @@ describe('SceneManager properties', () => {
       sceneManager.singleLayerMode = true;
 
       expect(sceneManager.singleLayerMode).toBe(true);
-      expect(sceneManager.startLayer).toBe(1);
+      expect(sceneManager.startLayer).toBe(2);
+    });
+
+    test('singleLayerMode clips away the layer below the end layer', () => {
+      sceneManager.endLayer = 2;
+
+      sceneManager.singleLayerMode = true;
+
+      const layer = sceneManager.job.layers[1];
+      const planes = objectsManager(sceneManager).clippingPlanes;
+      expect(planes).toHaveLength(1);
+      expect(planes[0].constant).toBe(-(layer.z - layer.height));
     });
 
     test('singleLayerMode restores the previous start layer when turned off', () => {
@@ -825,11 +836,15 @@ describe('SceneManager properties', () => {
       fresh.dispose();
     });
 
-    test('endLayer follows the end layer while single layer mode is on', () => {
+    test('startLayer follows the end layer while single layer mode is on', () => {
       sceneManager.singleLayerMode = true;
 
       sceneManager.endLayer = 2;
 
+      expect(sceneManager.startLayer).toBe(2);
+
+      sceneManager.endLayer = 1;
+
       expect(sceneManager.startLayer).toBe(1);
     });
   });

src/scene-manager.ts

@@ -519,7 +519,7 @@ export class SceneManager {
     }
 
     if (this._singleLayerMode === true) {
-      this.startLayer = this._endLayer - 1;
+      this.startLayer = this._endLayer;
     }
 
     this.updateClippingPlanes();
@@ -546,7 +546,7 @@ export class SceneManager {
 
     if (this._singleLayerMode) {
       this.prevStartLayer = this._startLayer;
-      this._startLayer = Math.max(this._endLayer - 1, 1);
+      this._startLayer = Math.max(this._endLayer, 1);
     } else {
       this._startLayer = this.prevStartLayer;
     }