Bug 772679. Make test_offsets.js handle the viewport being transformed. r=tnikkel

dom/tests/mochitest/general/test_offsets.js

@@ -119,11 +119,15 @@ function checkScrolledElement(element, child)
   element.scrollTop = 20;
   is(element.scrollLeft, 0, element.id + " scrollLeft after vertical scroll");
   is(element.scrollTop, 20, element.id + " scrollTop after vertical scroll");
-  is(child.getBoundingClientRect().top, childrect.top - 20, "child position after vertical scroll");
+  // If the viewport has been transformed, then we might have scrolled to a subpixel value
+  // that's slightly different from what we requested. After rounding, however, it should
+  // be the same.
+  is(Math.round(childrect.top - child.getBoundingClientRect().top), 20, "child position after vertical scroll");
 
   element.scrollTop = 0;
   is(element.scrollLeft, 0, element.id + " scrollLeft after vertical scroll reset");
   is(element.scrollTop, 0, element.id + " scrollTop after vertical scroll reset");
+  // Scrolling back to the top should work precisely.
   is(child.getBoundingClientRect().top, childrect.top, "child position after vertical scroll reset");
 
   element.scrollTop = 10;
@@ -135,7 +139,7 @@ function checkScrolledElement(element, child)
   element.scrollLeft = 18;
   is(element.scrollLeft, 18, element.id + " scrollLeft after horizontal scroll");
   is(element.scrollTop, 0, element.id + " scrollTop after horizontal scroll");
-  is(child.getBoundingClientRect().left, childrect.left - 18, "child position after horizontal scroll");
+  is(Math.round(childrect.left - child.getBoundingClientRect().left), 18, "child position after horizontal scroll");
 
   element.scrollLeft = -30;
   is(element.scrollLeft, 0, element.id + " scrollLeft after horizontal scroll reset");