Fix error in determining the target position for smooth scroll animation curve

third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html

Index: third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
diff --git a/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html b/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
index 702dbf148ac7116b0dac2896aac2bf5a3e1bed4f..2150050e13937e2c0782aebd586f0a3585210362 100644
--- a/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
+++ b/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/mousewheel-scroll.html
@@ -15,6 +15,21 @@
         smooth scrolling on the compositor works as intended (tested via\
         virtual suite virtual/threaded/).");
 
+    function testDiagonalScroll() {
+        // Reset
+        document.scrollingElement.scrollTop = 0;
+        document.scrollingElement.scrollLeft = 0;
+
+        eventSender.mouseMoveTo(20, 20);
+        // Scroll 3 ticks diagonally.
+        eventSender.mouseScrollBy(-3, -3);
+        // Undo 1 tick in each direction.
+        eventSender.mouseScrollBy(1, 1);
+        // 40px per tick.
+        shouldBecomeEqual("document.scrollingElement.scrollTop == 80 && " +

[Rick Byers, 2015/12/23 19:13:33]

Should this perhaps use different expected scroll position values than the first
scroll?  That way if the test is broken and no scrolling happens at all for this
second test, it'll fail instead of just maintaining the value from the first
test?

[ymalik, 2015/12/23 19:20:37]

Done.

+          "document.scrollingElement.scrollLeft == 80", "true", finishJSTest);
+    }
+
     function runTest() {
         if (!window.eventSender || !window.internals) {
             finishJSTest();
@@ -27,19 +42,17 @@
         eventSender.mouseMoveTo(20, 20);
         // Scroll down 3 ticks.
         eventSender.mouseScrollBy(0, -1);
-        eventSender.mouseScrollBy(0, -1);
-        eventSender.mouseScrollBy(0, -1);
+        eventSender.mouseScrollBy(0, -2);
         // Scroll right 3 ticks.
         eventSender.mouseScrollBy(-1, 0);
-        eventSender.mouseScrollBy(-1, 0);
-        eventSender.mouseScrollBy(-1, 0);
+        eventSender.mouseScrollBy(-2, 0);
         // Undo 1 tick in each direction.
         eventSender.mouseScrollBy(0, 1);
         eventSender.mouseScrollBy(1, 0);
 
         // 40px per tick.
         shouldBecomeEqual("document.scrollingElement.scrollTop == 80 && " +
-          "document.scrollingElement.scrollLeft == 80", "true", finishJSTest);
+          "document.scrollingElement.scrollLeft == 80", "true", testDiagonalScroll);
     }
 </script>