Preserve fractional scroll offset for JS scrolling API

LayoutTests/fast/scrolling/scrolling-apis-subpixel.html

Index: LayoutTests/fast/scrolling/scrolling-apis-subpixel.html
diff --git a/LayoutTests/fast/scrolling/scrolling-apis-subpixel.html b/LayoutTests/fast/scrolling/scrolling-apis-subpixel.html
index a3db4d0ed0a9e39d6a56644a9d79d5035f1bbb05..04d7f8a45a77cedea8b6e2c35c049bea649259fa 100644
--- a/LayoutTests/fast/scrolling/scrolling-apis-subpixel.html
+++ b/LayoutTests/fast/scrolling/scrolling-apis-subpixel.html
@@ -28,13 +28,15 @@ description("Verifies that scrolling APIs support fractional offsets.");
 // browser zoom.  When http://crbug.com/414283 is fixed, we should test
 // other cases like device scale.
 
+var floatPrecision = 0.01;

[Rick Byers, 2014/10/02 21:44:05]

How close is this in practice?  Eg. would 0.0001 also work?

I'm worried that there may be web page bugs if we're off by a non-trivial
amount.  I believe we have some cases where we convert from double to LayoutUnit
and back to double again (which could be responsible for differences of ~0.01) -
those probably need to be considered bugs and fixed to consistently use double.

[Yufeng Shen (Slow to review), 2014/10/02 23:34:34]

Yes. Element::setScrollLeft() has a conversion from float to LayoutUnit
LayoutUnit::fromFloatRound() which adds about ~0.003. So that's why I have to
put the precision so large, e.g. if you call setScrollLeft(4.2) and
read it back, it would be something like 4.203.

I am leaving a FIXME here to make the floatPrecision smaller.

[Rick Byers, 2014/10/03 16:42:46]

Ok.  Ideally we'll even be able to go back to shouldBeEqualToNumber (if all
we're doing is reading the exact double we stored, there should be no numerics
issues).

+
 function testScroll(scrollOffset) {
 
     debug('Scrolling DIV with scrollTop/scrollLeft');
     scroller.scrollTop = scrollOffset;
-    shouldBeEqualToNumber('scroller.scrollTop', scrollOffset);
+    shouldBeCloseTo('scroller.scrollTop', scrollOffset, floatPrecision);
     scroller.scrollLeft = scrollOffset;
-    shouldBeEqualToNumber('scroller.scrollLeft', scrollOffset);
+    shouldBeCloseTo('scroller.scrollLeft', scrollOffset, floatPrecision);
 
     // Note that the body element is a special case - we don't attempt to
     // test it here as it's semantics are in flux (http://goo.gl/BFHtMR).
@@ -43,20 +45,20 @@ function testScroll(scrollOffset) {
     window.scroll(0,0);
     scrollOffset++;
     window.scroll(scrollOffset, scrollOffset);
-    shouldBeEqualToNumber('window.scrollY', scrollOffset);
-    shouldBeEqualToNumber('window.scrollX', scrollOffset);
+    shouldBeCloseTo('window.scrollY', scrollOffset, floatPrecision);
+    shouldBeCloseTo('window.scrollX', scrollOffset, floatPrecision);
 
     debug('Scrolling the document with window.scrollTo');
     window.scroll(0,0);
     window.scrollTo(scrollOffset, scrollOffset);
-    shouldBeEqualToNumber('window.pageYOffset', scrollOffset);
-    shouldBeEqualToNumber('window.pageXOffset', scrollOffset);
+    shouldBeCloseTo('window.pageYOffset', scrollOffset, floatPrecision);
+    shouldBeCloseTo('window.pageXOffset', scrollOffset, floatPrecision);
 
     debug('Scrolling the document with window.scrollBy');
     window.scroll(1,1);
     window.scrollBy(scrollOffset - 1, scrollOffset - 1);
-    shouldBeEqualToNumber('window.scrollY', scrollOffset);
-    shouldBeEqualToNumber('window.scrollX', scrollOffset);
+    shouldBeCloseTo('window.scrollY', scrollOffset, floatPrecision);
+    shouldBeCloseTo('window.scrollX', scrollOffset, floatPrecision);
 
     debug('');
 }
@@ -68,5 +70,6 @@ function testPageZoom(zoom) {
     testScroll(4.5);
 }
 
+testScroll(4.2);
 testPageZoom(2);
 </script>