Bug 1300611 - Move overflow rect into correct coordinate space when computing perspective overflows so that it's not affected by scroll position. r=dbaron

layout/generic/nsGfxScrollFrame.cpp

@@ -947,7 +947,7 @@ GetScrollableOverflowForPerspective(nsIFrame* aScrolledFrame,
         // 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.

layout/generic/test/test_bug1198135.html

@@ -77,8 +77,13 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1198135
 
 <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>