Mac: Support Sierra's broken MouseWheel events.

ui/events/cocoa/events_mac.mm

Index: ui/events/cocoa/events_mac.mm
diff --git a/ui/events/cocoa/events_mac.mm b/ui/events/cocoa/events_mac.mm
index 4a8347fd25f67bb93125ef6f5ff82f407334ee13..4c334682c71f18536c8b0cf7438f4461f191b8f1 100644
--- a/ui/events/cocoa/events_mac.mm
+++ b/ui/events/cocoa/events_mac.mm
@@ -155,8 +155,26 @@ gfx::Vector2d GetMouseWheelOffset(const base::NativeEvent& event) {
     // Use the same multiplier as content::WebMouseWheelEventBuilder. Note this
     // differs from the value returned by CGEventSourceGetPixelsPerLine(), which
     // is typically 10.
-    return gfx::Vector2d(kScrollbarPixelsPerCocoaTick * [event deltaX],
-                         kScrollbarPixelsPerCocoaTick * [event deltaY]);
+    CGFloat deltaX = [event deltaX] * kScrollbarPixelsPerCocoaTick;
+    CGFloat deltaY = [event deltaY] * 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;
+    int64_t pointDeltaY = CGEventGetIntegerValueField(
+          [event CGEvent], kCGScrollWheelEventPointDeltaAxis1);
+    if (deltaY == 0 && pointDeltaY != 0)
+      deltaY = pointDeltaY * kPixelsPerLine;
+
+    return gfx::Vector2d(deltaX, deltaY);
   }
 }