diff --git a/dom/tests/mochitest/general/chrome.ini b/dom/tests/mochitest/general/chrome.ini
index b6b023b42eb..d34475d2706 100644
--- a/dom/tests/mochitest/general/chrome.ini
+++ b/dom/tests/mochitest/general/chrome.ini
@@ -6,3 +6,4 @@ skip-if = buildapp == 'b2g' || os == 'android'
 [test_offsets.js]
 [test_offsets.xul]
 skip-if = buildapp == 'mulet'
+[test_spacetopagedown.html]
diff --git a/dom/tests/mochitest/general/test_spacetopagedown.html b/dom/tests/mochitest/general/test_spacetopagedown.html
new file mode 100644
index 00000000000..03f840f3d68
--- /dev/null
+++ b/dom/tests/mochitest/general/test_spacetopagedown.html
@@ -0,0 +1,76 @@
+<html>
+<head>
+  <meta charset="utf-8">
+  <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"/>
+  <script type="application/javascript;version=1.7">
+
+SimpleTest.waitForExplicitFinish();
+
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+var windowUtils = SpecialPowers.getDOMWindowUtils(window);
+
+function pressKey(isShift)
+{
+  return new Promise(resolve => {
+    synthesizeKey(" ", { shiftKey: isShift });
+    windowUtils.advanceTimeAndRefresh(100);
+    SimpleTest.executeSoon(resolve);
+  });
+}
+
+function initTest()
+{
+  SpecialPowers.pushPrefEnv({"set":[["general.smoothScroll", false]]}, runTest);
+}
+
+function runTest()
+{
+  Task.async(function () {
+    yield pressKey(false);
+
+    ok(window.scrollY > 0, "Space with no focus" + window.scrollY);
+    yield pressKey(true);
+    is(window.scrollY, 0, "Shift+Space with no focus");
+
+    let checkbox = document.getElementById("checkbox");
+    checkbox.focus();
+    yield pressKey(false);
+
+    is(window.scrollY, 0, "Space with checkbox focused");
+    ok(checkbox.checked, "Space with checkbox focused, checked");
+    yield pressKey(true);
+    is(window.scrollY, 0, "Shift+Space with checkbox focused");
+    ok(!checkbox.checked, "Space with checkbox focused, unchecked");
+
+    let input = document.getElementById("input");
+    input.focus();
+    yield pressKey(false);
+    is(window.scrollY, 0, "Space with input focused");
+    is(input.value, " ", "Space with input focused, value");
+    yield pressKey(true);
+    is(window.scrollY, 0, "Shift+Space with input focused");
+    is(input.value, "  ", "Space with input focused, value");
+
+    windowUtils.restoreNormalRefresh();
+    SimpleTest.finish();
+  })();
+}
+
+  </script>
+</head>
+<body onload="SimpleTest.waitForFocus(initTest)">
+
+<input id="checkbox" type="checkbox">Checkbox
+<input id="input">
+<p style="height: 4000px">Text</p>
+
+<p id="display"></p>
+<div id="content" style="display: none">
+</div>
+<pre id="test">
+</pre>
+</body>
+</html>
diff --git a/dom/xbl/nsXBLPrototypeHandler.cpp b/dom/xbl/nsXBLPrototypeHandler.cpp
index 4cba65f280b..be2120918ac 100644
--- a/dom/xbl/nsXBLPrototypeHandler.cpp
+++ b/dom/xbl/nsXBLPrototypeHandler.cpp
@@ -459,6 +459,10 @@ nsXBLPrototypeHandler::DispatchXBLCommand(EventTarget* aTarget, nsIDOMEvent* aEv
   else
     controller = GetController(aTarget); // We're attached to the receiver possibly.
 
+  // We are the default action for this command.
+  // Stop any other default action from executing.
+  aEvent->PreventDefault();
+
   if (mEventName == nsGkAtoms::keypress &&
       mDetail == nsIDOMKeyEvent::DOM_VK_SPACE &&
       mMisc == 1) {
@@ -491,10 +495,6 @@ nsXBLPrototypeHandler::DispatchXBLCommand(EventTarget* aTarget, nsIDOMEvent* aEv
       }
     }
   }
-
-  // We are the default action for this command.
-  // Stop any other default action from executing.
-  aEvent->PreventDefault();
   
   if (controller)
     controller->DoCommand(command.get());
