diff --git a/browser/components/downloads/test/browser/Makefile.in b/browser/components/downloads/test/browser/Makefile.in
index 39e02866717..71dc291f6c3 100644
--- a/browser/components/downloads/test/browser/Makefile.in
+++ b/browser/components/downloads/test/browser/Makefile.in
@@ -4,5 +4,6 @@
 MOCHITEST_BROWSER_FILES = \
   browser_basic_functionality.js \
   browser_first_download_panel.js \
+  browser_overflow_anchor.js \
   head.js \
   $(NULL)
diff --git a/browser/components/downloads/test/browser/browser_overflow_anchor.js b/browser/components/downloads/test/browser/browser_overflow_anchor.js
new file mode 100644
index 00000000000..682c39ffd96
--- /dev/null
+++ b/browser/components/downloads/test/browser/browser_overflow_anchor.js
@@ -0,0 +1,115 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Make sure the downloads button and indicator overflows into the nav-bar
+ * chevron properly, and then when those buttons are clicked in the overflow
+ * panel that the downloads panel anchors to the chevron.
+ */
+function test_task() {
+  try {
+    // Ensure that state is reset in case previous tests didn't finish.
+    yield task_resetState();
+
+    // Record the original width of the window so we can put it back when
+    // this test finishes.
+    let oldWidth = window.outerWidth;
+
+    // The downloads button should not be overflowed to begin with.
+    let button = CustomizableUI.getWidget("downloads-button")
+                               .forWindow(window);
+    ok(!button.overflowed, "Downloads button should not be overflowed.");
+
+    // Hack - we lock the size of the default flex-y items in the nav-bar,
+    // namely, the URL and search inputs. That way we can resize the
+    // window without worrying about them flexing.
+    const kFlexyItems = ["urlbar-container", "search-container"];
+    registerCleanupFunction(() => unlockWidth(kFlexyItems));
+    lockWidth(kFlexyItems);
+
+    // Resize the window to half of its original size. That should
+    // be enough to overflow the downloads button.
+    window.resizeTo(oldWidth / 2, window.outerHeight);
+    yield waitForOverflowed(button, true);
+
+    let promise = promisePanelOpened();
+    button.node.doCommand();
+    yield promise;
+
+    let panel = DownloadsPanel.panel;
+    let chevron = document.getElementById("nav-bar-overflow-button");
+    is(panel.anchorNode, chevron, "Panel should be anchored to the chevron.");
+
+    DownloadsPanel.hidePanel();
+
+    // Unlock the widths on the flex-y items.
+    unlockWidth(kFlexyItems);
+
+    // Put the window back to its original dimensions.
+    window.resizeTo(oldWidth, window.outerHeight);
+
+    // The downloads button should eventually be un-overflowed.
+    yield waitForOverflowed(button, false);
+
+    // Now try opening the panel again.
+    promise = promisePanelOpened();
+    button.node.doCommand();
+    yield promise;
+
+    is(panel.anchorNode.id, "downloads-indicator-anchor");
+
+    DownloadsPanel.hidePanel();
+  } finally {
+    // Clean up when the test finishes.
+    yield task_resetState();
+  }
+}
+
+/**
+ * For some node IDs, finds the nodes and sets their min-width's to their
+ * current width, preventing them from flex-shrinking.
+ *
+ * @param aItemIDs an array of item IDs to set min-width on.
+ */
+function lockWidth(aItemIDs) {
+  for (let itemID of aItemIDs) {
+    let item = document.getElementById(itemID);
+    let curWidth = item.getBoundingClientRect().width + "px";
+    item.style.minWidth = curWidth;
+  }
+}
+
+/**
+ * Clears the min-width's set on a set of IDs by lockWidth.
+ *
+ * @param aItemIDs an array of ItemIDs to remove min-width on.
+ */
+function unlockWidth(aItemIDs) {
+  for (let itemID of aItemIDs) {
+    let item = document.getElementById(itemID);
+    item.style.minWidth = "";
+  }
+}
+
+/**
+ * Waits for a node to enter or exit the overflowed state.
+ *
+ * @param aItem the node to wait for.
+ * @param aIsOverflowed if we're waiting for the item to be overflowed.
+ */
+function waitForOverflowed(aItem, aIsOverflowed) {
+  let deferOverflow = Promise.defer();
+  if (aItem.overflowed == aIsOverflowed) {
+    return deferOverflow.resolve();
+  }
+
+  let observer = new MutationObserver(function(aMutations) {
+    if (aItem.overflowed == aIsOverflowed) {
+      observer.disconnect();
+      deferOverflow.resolve();
+    }
+  });
+  observer.observe(aItem.node, {attributes: true});
+
+  return deferOverflow.promise;
+}