Mac: Support Sierra's broken MouseWheel events.

content/browser/renderer_host/input/web_input_event_builders_mac.mm

Index: content/browser/renderer_host/input/web_input_event_builders_mac.mm
diff --git a/content/browser/renderer_host/input/web_input_event_builders_mac.mm b/content/browser/renderer_host/input/web_input_event_builders_mac.mm
index 45c37416b0d02e754e0700ceaed613b8fbff8bc2..745c6d939fe9d4817ec5a47f749cdf1b6db12bbf 100644
--- a/content/browser/renderer_host/input/web_input_event_builders_mac.mm
+++ b/content/browser/renderer_host/input/web_input_event_builders_mac.mm
@@ -551,17 +551,35 @@ blink::WebMouseWheelEvent WebMouseWheelEventBuilder::Build(
   // the point delta data instead, since we cannot distinguish trackpad data
   // from data from any other continuous device.
 
-  if (CGEventGetIntegerValueField(cg_event, kCGScrollWheelEventIsContinuous)) {
-    result.deltaX = CGEventGetIntegerValueField(
+  int64_t pointDeltaX = CGEventGetIntegerValueField(
         cg_event, kCGScrollWheelEventPointDeltaAxis2);
-    result.deltaY = CGEventGetIntegerValueField(
+  int64_t pointDeltaY = CGEventGetIntegerValueField(
         cg_event, kCGScrollWheelEventPointDeltaAxis1);
-    result.wheelTicksX = result.deltaX / ui::kScrollbarPixelsPerCocoaTick;
-    result.wheelTicksY = result.deltaY / ui::kScrollbarPixelsPerCocoaTick;
+
+  if (CGEventGetIntegerValueField(cg_event, kCGScrollWheelEventIsContinuous)) {
+    result.deltaX = pointDeltaX;
+    result.deltaY = pointDeltaY;
+    result.wheelTicksX = pointDeltaX / ui::kScrollbarPixelsPerCocoaTick;
+    result.wheelTicksY = pointDeltaY / ui::kScrollbarPixelsPerCocoaTick;
     result.hasPreciseScrollingDeltas = true;
   } else {
     result.deltaX = [event deltaX] * ui::kScrollbarPixelsPerCocoaTick;
     result.deltaY = [event deltaY] * ui::kScrollbarPixelsPerCocoaTick;
+
+    // A rounding bug on Sierra means that single-tick wheel scrolls may be
+    // truncated to zero. Prefer kCGScrollWheelEventPointDeltaAxisXY in those
+    // cases. However, kCGScrollWheelEventPointDeltaAxisXY has a "nasty bug"
+    // when holding Shift (see "Of Mice and Men", above). So only check vertical
+    // scrolls. Also, to be consistent with the values when truncation does NOT
+    // scroll, it needs to be multiplied by some factor. The documentation for
+    // CGEventSourceGetPixelsPerLine() says "By default, the scale is about ten
+    // pixels per line." However, the actual relationship is fuzzy because of
+    // acceleration. Using a constant of 10 for small values seems to "feel
+    // right" when this bug is encountered. See http://crbug.com/660773.
+    const float kPixelsPerLine = 10.0;
+    if (result.deltaY == 0 && pointDeltaY != 0)
+      result.deltaY = pointDeltaY * kPixelsPerLine;
+
     result.wheelTicksY =
         CGEventGetIntegerValueField(cg_event, kCGScrollWheelEventDeltaAxis1);
     result.wheelTicksX =