UX: improve mobile search

Improves usability of header search icon while user is already in full page search in mobile.

Currently, hitting search icon a second time empties input and does not scroll up to show search form.

This commit scrolls up to show form and sets focus on input.

test/javascripts/acceptance/search-mobile-test.js.es6

@@ -0,0 +1,30 @@
+import { acceptance } from "helpers/qunit-helpers";
+
+acceptance("Search - Mobile", { mobileView: true });
+
+QUnit.test("search", async assert => {
+  await visit("/");
+
+  await click("#search-button");
+
+  assert.ok(
+    exists("input.full-page-search"),
+    "it shows the full page search form"
+  );
+  assert.ok(!exists(".search-results .fps-topic"), "no results by default");
+
+  await fillIn(".search-query", "posts");
+  await click(".search-cta");
+
+  assert.ok(find(".fps-topic").length === 1, "has one post");
+
+  await $(document).scrollTop(200);
+  await click("#search-button");
+
+  assert.equal(
+    find("input.full-page-search").val(),
+    "posts",
+    "it does not reset input when hitting search icon again"
+  );
+  assert.equal($(document).scrollTop(), 0, "it scrolls back to top of document");
+});