Fix zoom property crash after deleting a stop (#2153)

## Launch Checklist

<!-- Thanks for the PR! Feel free to add or remove items from the
checklist. -->

When a fixed paint value is converted to a zoom function, Maputnik
creates exactly two stops.

<img width="362" height="226" alt="image"
src="https://github.com/user-attachments/assets/209aebbf-7bb3-4422-b0c7-1019e20a9ddb"
/>

Deleting either one of those two stops leaves a single remaining value.
During the transition back to the ordinary field editor, `ZoomProperty`
can render with `stops` undefined. It previously called `.map`
unconditionally, causing the editor to crash.

<img width="2551" height="1248" alt="image"
src="https://github.com/user-attachments/assets/f5c2af59-dcae-4ffe-8245-f175eea9c843"
/>

This PR guards the stop list with optional chaining. The change only
protects the render path and does not alter zoom-function conversion or
value persistence.

Now, the remaining stop's value will be restored after deleting the
other stop:

<img width="367" height="168" alt="image"
src="https://github.com/user-attachments/assets/8d13236b-6140-451b-8613-0bc5f11893de"
/>

The regression test covers `circle-opacity`:

1. Convert a fixed value to a zoom function.
2. Change the second stop's value.
3. Delete the first of the two stops.
4. Verify that the ordinary field editor reappears.
5. Verify that the remaining stop's value is restored.
6. Edit the restored field and verify that the style updates
successfully.

 - [x] Briefly describe the changes in this PR.
 - [ ] Link to related issues.
- [x] Include before/after visuals or gifs if this PR includes visual
changes.
 - [x] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

CHANGELOG.md

@@ -8,6 +8,9 @@
 - _...Add new stuff here..._
 
 ### 🐞 Bug fixes
+- Prevent a crash when converting a function to an expression
+- Prevent a crash when deleting either stop from a function with two stops
+- Preserve expanded layer groups when deleting layers, including the first layer of a group
 - The map's data listener now fires on tile loads again, so source and vector layer field autocompletion is populated
 - The `maputnik` desktop binary now opens the default browser automatically on startup (opt out with `--no-browser`)
 - _...Add new stuff here..._

e2e/fixtures/grouped-layers-style.json

@@ -0,0 +1,16 @@
+{
+  "version": 8,
+  "name": "Grouped layers",
+  "sources": {},
+  "layers": [
+    { "id": "background", "type": "background" },
+    { "id": "park-a", "type": "background" },
+    { "id": "park-b", "type": "background" },
+    { "id": "park-c", "type": "background" },
+    { "id": "road", "type": "background" },
+    { "id": "park-d", "type": "background" },
+    { "id": "park-e", "type": "background" },
+    { "id": "water-a", "type": "background" },
+    { "id": "water-b", "type": "background" }
+  ]
+}

e2e/layer-editor.spec.ts

@@ -289,6 +289,20 @@ describe("layer editor", () => {
         });
       });
 
+      test("should leave a plain editable value after deleting a stop", async () => {
+        await when.setFunctionStopValue("circle-radius", "Output value", 1, "0");
+        await when.deleteFunctionStop("circle-radius");
+        await then(get.elementByTestId("spec-field:circle-radius")).shouldBeVisible();
+        await then(get.styleFromLocalStorage()).shouldDeepNestedInclude({
+          layers: [{ id, paint: { "circle-radius": 0 } }],
+        });
+
+        await when.setValue("spec-field-input:circle-radius", "7");
+        await then(get.styleFromLocalStorage()).shouldDeepNestedInclude({
+          layers: [{ id, paint: { "circle-radius": 7 } }],
+        });
+      });
+
       test("should set the base", async () => {
         await when.setFunctionBase("circle-radius", "2");
         await then(get.styleFromLocalStorage()).shouldDeepNestedInclude({
@@ -310,8 +324,9 @@ describe("layer editor", () => {
         });
       });
 
-      test("should convert to an expression", async () => {
+      test("should convert to an expression without crashing", async () => {
         await when.makeExpression("circle-radius");
+        await then(get.element("[data-wd-key='spec-field-container:circle-radius'] .maputnik-expression-editor")).shouldBeVisible();
         await then(get.styleFromLocalStorage()).shouldDeepNestedInclude({
           layers: [{ id, paint: { "circle-radius": ["interpolate", ["linear"], ["zoom"], 6, 5, 10, 5] } }],
         });

e2e/layers-list.spec.ts

@@ -306,6 +306,75 @@ describe("layers list", () => {
   });
 
   describe("groups", () => {
+    for (const control of ["menu", "trash"] as const) {
+      describe(`after deletion using ${control}`, () => {
+        beforeEach(async () => {
+          await when.setStyle("grouped_layers");
+        });
+
+        test("keeps all groups expanded when an earlier layer is deleted", async () => {
+          await when.click("skip-target-layer-list");
+          await then(get.elementByTestId("layer-list-item:park-b")).shouldBeVisible();
+          await when.deleteLayer("background", control);
+          await then(get.elementByTestId("layer-list-item:background")).shouldNotExist();
+          await then(get.elementByTestId("layer-list-group:park-0")).shouldExist();
+          await expect(get.elementByTestId("layer-list-group:water-6").getByRole("button")).toHaveAttribute("aria-expanded", "true");
+          await then(get.elementByTestId("layer-list-item:park-b")).shouldBeVisible();
+          await then(get.elementByTestId("layer-list-item:park-e")).shouldBeVisible();
+          await then(get.elementByTestId("layer-list-item:water-b")).shouldBeVisible();
+        });
+
+        test("preserves separate states for groups with the same prefix", async () => {
+          await when.click("layer-list-group:park-1");
+          await when.deleteLayer("background", control);
+          await then(get.elementByTestId("layer-list-group:park-0")).shouldExist();
+          await when.click("layer-list-item:road");
+          await expect(get.elementByTestId("layer-list-group:park-0").getByRole("button")).toHaveAttribute("aria-expanded", "true");
+          await expect(get.elementByTestId("layer-list-group:park-4").getByRole("button")).toHaveAttribute("aria-expanded", "false");
+          await then(get.elementByTestId("layer-list-item:park-b")).shouldBeVisible();
+          await then(get.elementByTestId("layer-list-item:park-e")).shouldNotBeVisible();
+        });
+
+        test("keeps a group expanded when its first layer is deleted", async () => {
+          await when.click("layer-list-group:park-1");
+          await when.deleteLayer("park-a", control);
+          await then(get.elementByTestId("layer-list-item:park-a")).shouldNotExist();
+          await when.click("layer-list-item:road");
+          await expect(get.elementByTestId("layer-list-group:park-1").getByRole("button")).toHaveAttribute("aria-expanded", "true");
+          await then(get.elementByTestId("layer-list-item:park-c")).shouldBeVisible();
+        });
+
+        for (const expandedGroup of ["park-1", "park-5"]) {
+          test(`keeps a merged group expanded when ${expandedGroup} was expanded`, async () => {
+            await when.click("layer-list-group:" + expandedGroup);
+            await when.deleteLayer("road", control);
+            await then(get.elementByTestId("layer-list-item:road")).shouldNotExist();
+            await expect(get.elementByTestId("layer-list-group:park-1").getByRole("button")).toHaveAttribute("aria-expanded", "true");
+            await then(get.elementByTestId("layer-list-item:park-b")).shouldBeVisible();
+            await then(get.elementByTestId("layer-list-item:park-e")).shouldBeVisible();
+          });
+        }
+
+        test("keeps a merged group collapsed when both groups were collapsed", async () => {
+          await when.deleteLayer("road", control);
+          await then(get.elementByTestId("layer-list-item:road")).shouldNotExist();
+          await when.click("layer-list-item:background");
+          await expect(get.elementByTestId("layer-list-group:park-1").getByRole("button")).toHaveAttribute("aria-expanded", "false");
+          await then(get.elementByTestId("layer-list-item:park-b")).shouldNotBeVisible();
+          await then(get.elementByTestId("layer-list-item:park-e")).shouldNotBeVisible();
+        });
+
+        test("preserves later groups after deleting an entire group", async () => {
+          await when.click("skip-target-layer-list");
+          await when.deleteLayer("park-a", control);
+          await when.deleteLayer("park-b", control);
+          await when.deleteLayer("park-c", control);
+          await expect(get.elementByTestId("layer-list-group:water-4").getByRole("button")).toHaveAttribute("aria-expanded", "true");
+          await then(get.elementByTestId("layer-list-item:water-b")).shouldBeVisible();
+        });
+      });
+    }
+
     test("simple", async () => {
       await when.setStyle("geojson");
 

e2e/maputnik-driver.ts

@@ -39,6 +39,7 @@ export class MaputnikDriver {
         "example-style-with-zoom-7-and-center-0-51.json",
         "example-style-with-zoom-5-and-center-50-50.json",
         "access-token-style.json",
+        "grouped-layers-style.json",
       ];
       for (const fixture of styleFixtures) {
         await this.helper.given.interceptAndMockResponse({
@@ -63,6 +64,18 @@ export class MaputnikDriver {
 
     modal: this.modalDriver.when,
 
+    deleteLayer: async (id: string, control: "menu" | "trash" = "menu") => {
+      if (control === "trash") {
+        await this.helper.when.hover("layer-list-item:" + id);
+        await this.helper.when.click("layer-list-item:" + id + ":delete");
+        return;
+      }
+
+      await this.helper.when.click("layer-list-item:" + id);
+      await this.helper.when.click("skip-target-layer-editor");
+      await this.helper.when.click("menu-delete-layer");
+    },
+
     setStyle: async (
       styleProperties:
         | "geojson"
@@ -73,6 +86,7 @@ export class MaputnikDriver {
         | "font"
         | "zoom_7_center_0_51"
         | "access_tokens"
+        | "grouped_layers"
         | "",
       zoom?: number
     ) => {
@@ -85,6 +99,7 @@ export class MaputnikDriver {
         font: "example-style-with-fonts.json",
         zoom_7_center_0_51: "example-style-with-zoom-7-and-center-0-51.json",
         access_tokens: "access-token-style.json",
+        grouped_layers: "grouped-layers-style.json",
       };
 
       const url = new URL(baseUrl);

package.json

@@ -29,16 +29,16 @@
   "dependencies": {
     "@codemirror/lang-json": "^6.0.2",
     "@codemirror/lint": "^6.9.7",
-    "@codemirror/state": "^6.7.2",
+    "@codemirror/state": "^6.7.4",
     "@codemirror/theme-one-dark": "^6.1.3",
-    "@codemirror/view": "^6.43.10",
+    "@codemirror/view": "^6.43.11",
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@mapbox/mapbox-gl-rtl-text": "^0.4.0",
     "@maplibre/maplibre-gl-geocoder": "^1.9.4",
     "@maplibre/maplibre-gl-inspect": "^1.9.0",
-    "@maplibre/maplibre-gl-style-spec": "^26.4.1",
+    "@maplibre/maplibre-gl-style-spec": "^26.4.2",
     "array-move": "^4.0.0",
     "buffer": "^6.0.3",
     "classnames": "^2.5.1",
@@ -48,7 +48,7 @@
     "downshift": "^9.4.0",
     "events": "^3.3.0",
     "file-saver": "^2.0.5",
-    "i18next": "^26.4.1",
+    "i18next": "^26.4.2",
     "i18next-browser-languagedetector": "^8.2.1",
     "i18next-resources-to-backend": "^1.2.3",
     "json-stringify-pretty-compact": "^4.0.0",
@@ -60,7 +60,7 @@
     "lodash.get": "^4.4.2",
     "lodash.isequal": "^4.5.0",
     "lodash.throttle": "^4.1.1",
-    "maplibre-gl": "^6.6.0",
+    "maplibre-gl": "^6.7.0",
     "maputnik-design": "github:maputnik/design#172b06c",
     "ol": "^10.10.0",
     "ol-mapbox-style": "^13.4.3",
@@ -101,7 +101,7 @@
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "@istanbuljs/nyc-config-typescript": "^1.0.2",
-    "@playwright/test": "^1.62.1",
+    "@playwright/test": "^1.63.0",
     "@stylistic/eslint-plugin": "^5.10.0",
     "@types/codemirror": "^5.60.18",
     "@types/color": "^4.2.1",
@@ -121,24 +121,24 @@
     "@types/react-aria-modal": "^5.0.0",
     "@types/react-collapse": "^5.0.4",
     "@types/react-color": "^3.0.13",
-    "@types/react-dom": "^19.2.5",
+    "@types/react-dom": "^19.2.7",
     "@types/string-hash": "^1.1.3",
     "@types/wicg-file-system-access": "^2023.10.7",
     "@vitejs/plugin-react": "^6.1.1",
-    "@vitest/coverage-v8": "^4.1.11",
+    "@vitest/coverage-v8": "^5.0.0",
     "cors": "^2.8.6",
-    "eslint": "^10.9.1",
+    "eslint": "^10.10.0",
     "eslint-plugin-react": "^7.37.5",
     "eslint-plugin-react-hooks": "^7.1.1",
-    "eslint-plugin-react-refresh": "^0.5.5",
+    "eslint-plugin-react-refresh": "^0.5.6",
-    "i18next-cli": "^1.72.1",
+    "i18next-cli": "^1.73.1",
     "istanbul": "^0.4.5",
     "istanbul-lib-coverage": "^3.2.2",
     "nyc": "^18.0.0",
-    "postcss": "^8.5.26",
+    "postcss": "^8.5.28",
     "react-hot-loader": "^4.13.1",
-    "sass": "^1.103.1",
+    "sass": "^1.104.0",
-    "stylelint": "^17.14.1",
+    "stylelint": "^17.15.0",
     "stylelint-config-recommended-scss": "^17.0.1",
     "stylelint-scss": "^7.2.0",
     "typescript": "^6.0.3",
@@ -146,6 +146,6 @@
     "uuid": "^14.0.2",
     "vite": "^8.2.2",
     "vite-plugin-istanbul": "^9.0.1",
-    "vitest": "^4.1.11"
+    "vitest": "^5.0.0"
   }
 }

src/components/FieldFunction.tsx

@@ -129,16 +129,10 @@ type FieldFunctionProps = {
  * https://www.mapbox.com/mapbox-gl-style-spec/#types-function-zoom-property
  */
 export const FieldFunction: React.FC<FieldFunctionProps> = (props) => {
-  const [dataType, setDataType] = React.useState(
-    getDataType(props.value, props.fieldSpec)
-  );
   const [isEditing, setIsEditing] = React.useState(false);
-
+  // Keep the expression editor mounted while typing, but otherwise select the
-  React.useEffect(() => {
+  // editor from the current value so a collapsed function never renders as stops.
-    if (!isEditing) {
+  const dataType = isEditing ? "expression" : getDataType(props.value, props.fieldSpec);
-      setDataType(getDataType(props.value, props.fieldSpec));
-    }
-  }, [props.value, props.fieldSpec, isEditing]);
 
   const getFieldFunctionType = (fieldSpec: any) => {
     if (fieldSpec.expression.interpolated) {
@@ -173,7 +167,7 @@ export const FieldFunction: React.FC<FieldFunctionProps> = (props) => {
   const deleteExpression = () => {
     const { fieldSpec, fieldName } = props;
     props.onChange(fieldName, fieldSpec.default);
-    setDataType("value");
+    setIsEditing(false);
   };
 
   const deleteStop = (stopIdx: number) => {
@@ -233,10 +227,10 @@ export const FieldFunction: React.FC<FieldFunctionProps> = (props) => {
         type: "identity",
         property: value[1],
       });
-      setDataType("value");
+      setIsEditing(false);
     } else if (isLiteralExpression(value)) {
       props.onChange(fieldName, value[1]);
-      setDataType("value");
+      setIsEditing(false);
     }
   };
 

src/components/LayerList.tsx

@@ -106,13 +106,13 @@ class LayerListContainerInternal extends React.Component<LayerListContainerInter
     });
   };
 
-  groupedLayers(): (LayerSpecification & {key: string})[][] {
+  groupedLayers(layers = this.props.layers): (LayerSpecification & {key: string})[][] {
     const groups = [];
     const layerIdCount = new Map();
 
-    for (let i = 0; i < this.props.layers.length; i++) {
+    for (let i = 0; i < layers.length; i++) {
-      const origLayer = this.props.layers[i];
+      const origLayer = layers[i];
-      const previousLayer = this.props.layers[i-1];
+      const previousLayer = layers[i-1];
       layerIdCount.set(origLayer.id,
         layerIdCount.has(origLayer.id) ? layerIdCount.get(origLayer.id) + 1 : 0
       );
@@ -192,7 +192,39 @@ class LayerListContainerInternal extends React.Component<LayerListContainerInter
     return propsChanged;
   }
 
+  /**
+   * Preserves group state through surviving layer IDs when group indices change.
+   * If deletion joins groups, the result stays expanded when either group was.
+   */
+  private preserveLayerGroupState(previousLayers: LayerSpecification[]) {
+    if (previousLayers === this.props.layers) return;
+
+    const collapsedByLayer = new Map<string, boolean>();
+    let idx = 0;
+    for (const layers of this.groupedLayers(previousLayers)) {
+      const collapsed = this.isCollapsed(layerPrefix(layers[0].id), idx);
+      for (const layer of layers) {
+        collapsedByLayer.set(layer.id, collapsed);
+      }
+      idx += layers.length;
+    }
+
+    const collapsedGroups: {[key: string]: boolean} = {};
+    idx = 0;
+    for (const layers of this.groupedLayers()) {
+      const lookupKey = [layerPrefix(layers[0].id), idx].join("-");
+      collapsedGroups[lookupKey] = layers.every(layer => collapsedByLayer.get(layer.id) !== false);
+      idx += layers.length;
+    }
+
+    if (!lodash.isEqual(collapsedGroups, this.state.collapsedGroups)) {
+      this.setState({ collapsedGroups });
+    }
+  }
+
   componentDidUpdate (prevProps: LayerListContainerProps) {
+    this.preserveLayerGroupState(prevProps.layers);
+
     if (prevProps.selectedLayerIndex !== this.props.selectedLayerIndex) {
       const selectedItemNode = this.selectedItemRef.current;
       if (selectedItemNode && selectedItemNode.node) {