diff --git a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
index d69dcae43f2b..d0ec0d6100cd 100644
--- a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
@@ -241,3 +241,23 @@ function synthesizeNativeClick(aElement, aX, aY, aObserver = null) {
   });
   return true;
 }
+
+// Move the mouse to (dx, dy) relative to |element|, and scroll the wheel
+// at that location.
+// Moving the mouse is necessary to avoid wheel events from two consecutive
+// scrollWheelOver() calls on different elements being incorreclty considered
+// as part of t he same wheel transaction.
+// We also wait for the mouse move event to be processed before sending the
+// wheel event, otherwise there is a chance they might get reordered, and
+// we have the transaction problem again.
+// XXX FOOTGUN: On Windows, if the mouse cursor is already at the target
+//              position, the mouse-move event doesn't get dispatched, and
+//              we end up hanging waiting for it. As a result, care must
+//              be taken that the mouse isn't already at the target position
+//              when using this function.
+function moveMouseAndScrollWheelOver(element, dx, dy, testDriver) {
+  return synthesizeNativeMouseMoveAndWaitForMoveEvent(element, dx, dy, function() {
+    synthesizeNativeWheelAndWaitForScrollEvent(element, dx, dy, 0, -10, testDriver);
+  });
+}
+
diff --git a/gfx/layers/apz/test/mochitest/helper_scroll_on_position_fixed.html b/gfx/layers/apz/test/mochitest/helper_scroll_on_position_fixed.html
index 87ed4000a12d..12f9cb001bbc 100644
--- a/gfx/layers/apz/test/mochitest/helper_scroll_on_position_fixed.html
+++ b/gfx/layers/apz/test/mochitest/helper_scroll_on_position_fixed.html
@@ -6,45 +6,31 @@
   <script type="application/javascript" src="/tests/SimpleTest/paint_listener.js"></script>
   <script type="application/javascript">
 
-// Scroll the mouse wheel at (dx, dy) relative to |element|.
-function scrollWheelOver(element, dx, dy, testDriver) {
-  // Move the mouse to the desired wheel location.
-  // Not doing so can result in the wheel events from two consecutive
-  // scrollWheelOver() calls on different elements being incorrectly considered
-  // as part of the same wheel transaction.
-  // We also wait for the mouse move event to be processed before sending the
-  // wheel event, otherwise there is a chance they might get reordered, and
-  // we have the transaction problem again.
-  return synthesizeNativeMouseMoveAndWaitForMoveEvent(element, dx, dy, function() {
-    synthesizeNativeWheelAndWaitForScrollEvent(element, dx, dy, 0, -10, testDriver);
-  });
-}
-
 function* test(testDriver) {
   var iframeWin = document.getElementById('iframe').contentWindow;
 
   // scroll over the middle of the iframe's position:sticky element, check
   // that it scrolls the iframe
   var scrollPos = iframeWin.scrollY;
-  yield scrollWheelOver(iframeWin.document.body, 50, 150, testDriver);
+  yield moveMouseAndScrollWheelOver(iframeWin.document.body, 50, 150, testDriver);
   ok(iframeWin.scrollY > scrollPos, "iframe scrolled after wheeling over the position:sticky element");
 
   // same, but using the iframe's position:fixed element
   scrollPos = iframeWin.scrollY;
-  yield scrollWheelOver(iframeWin.document.body, 250, 150, testDriver);
+  yield moveMouseAndScrollWheelOver(iframeWin.document.body, 250, 150, testDriver);
   ok(iframeWin.scrollY > scrollPos, "iframe scrolled after wheeling over the position:fixed element");
 
   // same, but scrolling the scrollable frame *inside* the position:fixed item
   var fpos = document.getElementById('fpos_scrollable');
   scrollPos = fpos.scrollTop;
-  yield scrollWheelOver(fpos, 50, 150, testDriver);
+  yield moveMouseAndScrollWheelOver(fpos, 50, 150, testDriver);
   ok(fpos.scrollTop > scrollPos, "scrollable item inside fixed-pos element scrolled");
   // wait for it to layerize fully and then try again
   yield waitForAllPaints(function() {
     flushApzRepaints(testDriver);
   });
   scrollPos = fpos.scrollTop;
-  // The mouse is already at the right position. If we call scrollWheelOver it
+  // The mouse is already at the right position. If we call moveMouseAndScrollWheelOver it
   // hangs on windows waiting for the mouse-move, so instead we just synthesize
   // the wheel directly.
   yield synthesizeNativeWheelAndWaitForScrollEvent(fpos, 50, 150, 0, -10, testDriver);
@@ -52,12 +38,12 @@ function* test(testDriver) {
 
   // same, but using the top-level window's position:sticky element
   scrollPos = window.scrollY;
-  yield scrollWheelOver(document.body, 50, 150, testDriver);
+  yield moveMouseAndScrollWheelOver(document.body, 50, 150, testDriver);
   ok(window.scrollY > scrollPos, "top-level document scrolled after wheeling over the position:sticky element");
 
   // same, but using the top-level window's position:fixed element
   scrollPos = window.scrollY;
-  yield scrollWheelOver(document.body, 250, 150, testDriver);
+  yield moveMouseAndScrollWheelOver(document.body, 250, 150, testDriver);
   ok(window.scrollY > scrollPos, "top-level document scrolled after wheeling over the position:fixed element");
 }