feat: make sidebar panels resizable (#2094)

- Replaces #1870, which replaced #1682, #1677

This mostly uses the code from there and addresses the comments I've
left. I didn't change the code much.

## Launch Checklist

Makes the sidebar panel resizable

<img width="707" height="511" alt="image"
src="https://github.com/user-attachments/assets/d010df38-faac-441c-bf7f-a4960e471c61"
/>


 - [x] Briefly describe the changes in this PR.
 - [x] 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.

---------

Co-authored-by: Bart Louwers <bart.louwers@gmail.com>

e2e/playwright-helper.ts

@@ -122,6 +122,8 @@ export class Assertable<T> {
   // Value assertions (auto-retrying for Query targets).
   shouldEqual = (value: any) => this.assertValue((actual) => expect(actual).toBe(value));
 
+  shouldBeGreaterThan = (value: number) => this.assertValue((actual) => expect(actual).toBeGreaterThan(value));
+
   shouldInclude = (value: any) =>
     this.assertValue((actual) => {
       if (typeof value === "object" && value !== null) {
@@ -349,6 +351,15 @@ export class PlaywrightHelper {
       await this.page.mouse.up();
     },
 
+    /** Presses at the centre of an element and drags it by the given offset. */
+    dragBy: async (testId: string, deltaX: number, deltaY = 0) => {
+      const { x, y } = await centerOf(this.testId(testId));
+      await this.page.mouse.move(x, y);
+      await this.page.mouse.down();
+      await this.page.mouse.move(x + deltaX, y + deltaY, { steps: 10 });
+      await this.page.mouse.up();
+    },
+
     clickCenter: async (testId: string) => {
       const { x, y } = await centerOf(this.testId(testId));
       await this.page.mouse.move(x, y);
@@ -446,6 +457,13 @@ export class PlaywrightHelper {
 
     inputValue: (testId: string) => new Query<string>(() => this.testId(testId).first().inputValue()),
 
+    elementWidth: (testId: string) =>
+      new Query<number>(async () => {
+        const box = await this.testId(testId).first().boundingBox();
+        if (!box) throw new Error(`Element "${testId}" has no bounding box`);
+        return box.width;
+      }),
+
     elementsText: (testId: string) => new Query<string>(() => this.testId(testId).first().innerText()),
 
     locationHash: () => new Query<string>(async () => new URL(this.page.url()).hash),

e2e/sidebar-resize.spec.ts

@@ -0,0 +1,27 @@
+import { beforeEach, describe, test } from "./utils/fixtures";
+import { MaputnikDriver } from "./maputnik-driver";
+
+describe("sidebar resize", () => {
+  const { given, get, when, then } = new MaputnikDriver();
+
+  beforeEach(async () => {
+    await given.setupMockBackedResponses();
+    await when.setStyle("layer");
+  });
+
+  test("dragging the outer handle widens the sidebar", async () => {
+    const initialWidth = await get.elementWidth("sidebar-panel").get();
+
+    await when.dragBy("sidebar-resize-handle", 100);
+
+    await then(get.elementWidth("sidebar-panel")).shouldBeGreaterThan(initialWidth + 50);
+  });
+
+  test("dragging the inner handle widens the layer list", async () => {
+    const initialWidth = await get.elementWidth("layer-list-panel").get();
+
+    await when.dragBy("inner-resize-handle", 50);
+
+    await then(get.elementWidth("layer-list-panel")).shouldBeGreaterThan(initialWidth + 20);
+  });
+});