--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -942,17 +942,17 @@ GetScrollableOverflowForPerspective(nsIF
         overhang.top /= bottomDelta;
         overhang.right /= rightDelta;
         overhang.bottom /= bottomDelta;
         overhang.left /= rightDelta;
 
         // Take the minimum overflow rect that would allow the current scroll
         // position, using the size of the scroll port and offset by the
         // inverse of the scroll position.
-        nsRect overflow(0, 0, aScrollPort.width, aScrollPort.height);
+        nsRect overflow = aScrollPort - scrollPos;
 
         // Expand it by our margins to get an overflow rect that would allow all
         // edges of our transformed content to be scrolled into view.
         overflow.Inflate(overhang);
 
         // Merge it with the combined overflow
         aScrolledFrameOverflowArea.UnionRect(aScrolledFrameOverflowArea,
                                              overflow);

--- a/layout/generic/test/test_bug1198135.html
+++ b/layout/generic/test/test_bug1198135.html
@@ -72,13 +72,18 @@ https://bugzilla.mozilla.org/show_bug.cg
   <div class="example__group">
     <div class="example__layer layer--c"></div>
     <div class="example__layer layer--e"></div>
   </div>
 </div>
 
 <script>
   is(document.getElementById("first").scrollHeight, 600, "Scroll height should be computed correctly");
+  document.getElementById("first").scrollTop = 150;
+  is(document.getElementById("first").scrollHeight, 600, "Scroll height should be a constant when scrolling");
+
   // The true height is 727.5 and we don't always snap the same direction.
   isfuzzy(document.getElementById("second").scrollHeight, 728, 1, "Scroll height should be computed correctly");
+  document.getElementById("second").scrollTop = 150;
+  isfuzzy(document.getElementById("second").scrollHeight, 728, 1, "Scroll height should be a constant when scrolling");
 </script>
 
 </body></html>