Scroll option page for Firefox 60

e2e/lib/OptionPage.ts

@@ -32,5 +32,8 @@ export default class OptionPage {
     // TODO validate current page
     return new JSONOptionPage(this.lanthan);
   }
-}
 
+  scrollTo(x: number, y: number): Promise<void> {
+    return this.webdriver.executeScript(`window.scrollTo(${x}, ${y})`);
+  }
+}

e2e/options_form.test.ts

@@ -38,6 +38,8 @@ describe("options form page", () => {
   it('add blacklist', async () => {
     let page = await OptionPage.open(lanthan);
     let forms = await page.switchToForm();
+    // Scroll is required to click a button on Firefox 60
+    await page.scrollTo(0, 1000);
 
     // assert default
     let settings = (await browser.storage.local.get('settings')).settings;