fix(core): first track ID should not be lost after compile

src/compiler/compile.test.ts

@@ -2,6 +2,9 @@ import { EX_SPEC_VISUAL_ENCODING } from '../../editor/example/json-spec/visual-e
 import { compile } from './compile';
 import { getTheme } from '../core/utils/theme';
 import type { GoslingSpec } from '../index';
+import { convertToFlatTracks } from './spec-preprocess';
+import type { SingleView } from '@gosling-lang/gosling-schema';
+import { spreadTracksByData } from '../core/utils/overlay';
 
 describe('compile', () => {
     it('compile should not touch the original spec of users', () => {
@@ -242,3 +245,24 @@ describe('Dummy track', () => {
         );
     });
 });
+
+describe('Maintain IDs', () => {
+    it('Overlaid tracks', () => {
+        const twoTracksWithDiffData: SingleView = {
+            alignment: 'overlay',
+            tracks: [
+                {
+                    id: 'first',
+                    data: { type: 'csv', url: 'http://abc' }
+                },
+                {
+                    id: 'second',
+                    data: { type: 'csv', url: 'http://def' }
+                }
+            ]
+        };
+        const flattened = convertToFlatTracks(twoTracksWithDiffData);
+        const spread = spreadTracksByData(flattened);
+        expect(spread.map(d => d.id)).toEqual(['first', 'second']);
+    });
+});

src/core/gosling-component.tsx

@@ -86,7 +86,7 @@ export const GoslingComponent = forwardRef<GoslingRef, GoslingCompProps>((props,
      * This makes sure that all the current zooming status is preserved when new tracks are added
      */
     const preverseZoomStatus = (newSpec: gosling.HiGlassSpec, prevSpec: gosling.HiGlassSpec) => {
-        newSpec.views.forEach((view) => {
+        newSpec.views.forEach(view => {
             const viewUid = view.uid!;
             const newView = !prevSpec.views.find(v => v.uid === viewUid);
             if (newView) {

src/core/utils/overlay.ts

@@ -95,6 +95,11 @@ export function spreadTracksByData(tracks: Track[]): Track[] {
                     original.data = subSpec.data;
                 }
 
+                // If the id is undefined, put the first spec to the parent
+                if (!original.id) {
+                    original.id = subSpec.id;
+                }
+
                 // Determine if this `subSpec` should be added to `overlay` or become a separate track
                 if (!subSpec.data || isIdenticalDataSpec([original.data, subSpec.data])) {
                     original.overlay.push(subSpec);