1 files changed, 49 insertions, 0 deletions
diff --git a/devtools/client/styleeditor/test/browser_styleeditor_bug_870339.js b/devtools/client/styleeditor/test/browser_styleeditor_bug_870339.js
new file mode 100644
index 0000000000..926048223d
--- /dev/null
+++ b/devtools/client/styleeditor/test/browser_styleeditor_bug_870339.js
@@ -0,0 +1,49 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+const SIMPLE = TEST_BASE_HTTP + "simple.css";
+const DOCUMENT_WITH_ONE_STYLESHEET =
+  "data:text/html;charset=UTF-8," +
+  encodeURIComponent(
+    [
+      "<!DOCTYPE html>",
+      "<html>",
+      " <head>",
+      "  <title>Bug 870339</title>",
+      '  <link rel="stylesheet" type="text/css" href="' + SIMPLE + '">',
+      " </head>",
+      " <body>",
+      " </body>",
+      "</html>",
+    ].join("\n")
+  );
+
+add_task(async function () {
+  const { ui } = await openStyleEditorForURL(DOCUMENT_WITH_ONE_STYLESHEET);
+
+  // Spam the "devtools.source-map.client-service.enabled" pref observer callback (#onOrigSourcesPrefChanged)
+  // multiple times before the StyleEditorActor has a chance to respond to the first one.
+  const SPAM_COUNT = 2;
+  let prefValue = false;
+  for (let i = 0; i < SPAM_COUNT; ++i) {
+    pushPref("devtools.source-map.client-service.enabled", prefValue);
+    prefValue = !prefValue;
+  }
+
+  // Wait for the StyleEditorActor to respond to each pref changes.
+  await new Promise(resolve => {
+    let loadCount = 0;
+    ui.on("stylesheets-refreshed", function onReset() {
+      ++loadCount;
+      if (loadCount == SPAM_COUNT) {
+        ui.off("stylesheets-refreshed", onReset);
+        // No matter how large SPAM_COUNT is, the number of style
+        // sheets should never be more than the number of style sheets
+        // in the document.
+        is(ui.editors.length, 1, "correct style sheet count");
+        resolve();
+      }
+    });
+  });
+});