Bug 1400491 - Set up mutation observer before synthesizing composition. r=mikedeboer,ursula

Undisable test for mac opt and linux32 opt.

MozReview-Commit-ID: 74IZ6VkPdER

--HG--
extra : rebase_source : 72301600b2a52dc4b68d483dd675ea0b2d31dc75

browser/base/content/test/about/browser.ini

@@ -16,7 +16,6 @@ support-files =
 skip-if = true # Bug 1409054 to remove; previously skipped for intermittents, e.g., Bug 1399648
 [browser_aboutHome_search_POST.js]
 [browser_aboutHome_search_composing.js]
-skip-if = !debug && (os == "mac" || (os == "linux" && bits == 32)) # Bug 1400491, bug 1399648
 [browser_aboutHome_search_searchbar.js]
 [browser_aboutHome_search_suggestion.js]
 skip-if = os == "mac" || (os == "linux" && (!debug || bits == 64)) # Bug 1399648, bug 1402502

browser/base/content/test/about/browser_aboutHome_search_composing.js

@@ -21,18 +21,8 @@ add_task(async function() {
       input.focus();
     });
 
-    // FYI: "compositionstart" will be dispatched automatically.
-    await BrowserTestUtils.synthesizeCompositionChange({
-      composition: {
-        string: "x",
-        clauses: [
-          { length: 1, attr: Ci.nsITextInputProcessor.ATTR_RAW_CLAUSE }
-        ]
-      },
-      caret: { start: 1, length: 0 }
-    }, browser);
-
-    await ContentTask.spawn(browser, null, async function() {
+    info("Setting up the mutation observer before synthesizing composition");
+    let mutationPromise = ContentTask.spawn(browser, null, async function() {
       let searchController = content.wrappedJSObject.gContentSearchController;
 
       // Wait for the search suggestions to become visible.
@@ -63,6 +53,20 @@ add_task(async function() {
       searchController.selectedIndex = 1;
     });
 
+    // FYI: "compositionstart" will be dispatched automatically.
+    await BrowserTestUtils.synthesizeCompositionChange({
+      composition: {
+        string: "x",
+        clauses: [
+          { length: 1, attr: Ci.nsITextInputProcessor.ATTR_RAW_CLAUSE }
+        ]
+      },
+      caret: { start: 1, length: 0 }
+    }, browser);
+
+    info("Waiting for search suggestion table unhidden");
+    await mutationPromise;
+
     // Click the second suggestion.
     let expectedURL = Services.search.currentEngine
       .getSubmission("xbar", null, "homepage").uri.spec;