Increase coverage (#1997)

## Launch Checklist

This PR increases coverage by adding unit tests to lib folde, replace
the skipped end to end placeholder with actual tests and adds more end
to end tests.

This was mostly done by AI (Claude opus 4.8) and I reviewed it and
requested changes where needed.

 - [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: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

e2e/playwright-helper.ts

@@ -62,11 +62,12 @@ function isLocator(target: unknown): target is Locator {
   );
 }
 
-/** Asserts that every top-level key in `expected` deep-equals its counterpart in `actual`. */
-function assertDeepNestedInclude(actual: any, expected: Record<string, unknown>): void {
-  for (const key of Object.keys(expected)) {
-    expect(actual?.[key], `property "${key}"`).toEqual(expected[key]);
-  }
+/**
+ * Asserts that `actual` recursively contains everything in `expected`: nested
+ * objects are matched as subsets, while arrays and primitives must match exactly.
+ */
+function assertDeepNestedInclude(actual: any, expected: Record<string, unknown> | unknown[]): void {
+  expect(actual).toMatchObject(expected);
 }
 
 /**
@@ -130,7 +131,7 @@ export class Assertable<T> {
       }
     });
 
-  shouldDeepNestedInclude = (value: Record<string, unknown>) =>
+  shouldDeepNestedInclude = (value: Record<string, unknown> | unknown[]) =>
     this.assertValue((actual) => assertDeepNestedInclude(actual, value));
 }
 
@@ -144,6 +145,8 @@ async function typeSequence(page: Page, text: string): Promise<void> {
     del: "Delete",
     tab: "Tab",
     home: "Home",
+    end: "End",
+    rightarrow: "ArrowRight",
   };
 
   for (let i = 0; i < tokens.length; i++) {
@@ -202,10 +205,31 @@ export class PlaywrightHelper {
     return new Query<T>(getter);
   }
 
+  /** Stubs the File System Access "save" picker so file saves complete headlessly. */
+  public stubSaveFilePicker(): Promise<void> {
+    return this.page.evaluate(() => {
+      (window as any).showSaveFilePicker = async () => ({
+        createWritable: async () => ({ write: async () => {}, close: async () => {} }),
+      });
+    });
+  }
+
   /** Entry point for fluent assertions over a Locator or a value/Query. */
   public then = <T>(target: T): Assertable<T> => new Assertable(target);
 
   public given = {
+    /**
+     * Removes the File System Access API so the app falls back to a plain
+     * <input type="file">, the way Firefox and Safari behave. Must be called
+     * before the page under test is loaded.
+     */
+    noFileSystemAccessApi: async () => {
+      await this.page.addInitScript(() => {
+        delete (window as any).showOpenFilePicker;
+        delete (window as any).showSaveFilePicker;
+      });
+    },
+
     intercept: async (pattern: RegExp, alias: string, _method = "GET") => {
       this.recordedRequests.set(alias, []);
       await this.page.route(pattern, (route) => {
@@ -333,23 +357,38 @@ export class PlaywrightHelper {
       await this.page.mouse.up();
     },
 
-    openFileByFixture: async (fixture: string, buttonTestId: string, inputTestId: string) => {
+    /**
+     * Opens a fixture through the File System Access API, which raises no
+     * "filechooser" event and so has to be stubbed. For the <input type="file">
+     * fallback that browsers without the API use, see chooseFileFromPicker.
+     */
+    openFileByFixture: async (fixture: string, buttonTestId: string) => {
       const content = JSON.stringify(this.readFixture(fixture));
-      const hasPicker = await this.page.evaluate(() => "showOpenFilePicker" in window);
-      if (hasPicker) {
-        await this.page.evaluate((fileContent) => {
-          (window as any).showOpenFilePicker = async () => [
-            { getFile: async () => ({ text: async () => fileContent }) },
-          ];
-        }, content);
-        await this.testId(buttonTestId).click();
-      } else {
-        await this.testId(inputTestId).setInputFiles({
-          name: fixture,
-          mimeType: "application/json",
-          buffer: Buffer.from(content),
-        });
-      }
+      await this.page.evaluate((fileContent) => {
+        (window as any).showOpenFilePicker = async () => [
+          { getFile: async () => ({ text: async () => fileContent }) },
+        ];
+      }, content);
+      await this.testId(buttonTestId).click();
+    },
+
+    /**
+     * Clicks a control that opens the browser's native file chooser and answers
+     * it with a fixture. Only works on the <input type="file"> path — the File
+     * System Access API does not raise a "filechooser" event, so pair this with
+     * given.noFileSystemAccessApi().
+     */
+    chooseFileFromPicker: async (fixture: string, triggerTestId: string) => {
+      const content = JSON.stringify(this.readFixture(fixture));
+      const [fileChooser] = await Promise.all([
+        this.page.waitForEvent("filechooser"),
+        this.testId(triggerTestId).click(),
+      ]);
+      await fileChooser.setFiles({
+        name: fixture,
+        mimeType: "application/json",
+        buffer: Buffer.from(content),
+      });
     },
 
     dropFileByFixture: async (fixture: string, dropzoneTestId: string) => {