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..e2426620f424d4eefacf510ff64b12affb14b4e4 100644
--- a/ui/events/cocoa/events_mac.mm
+++ b/ui/events/cocoa/events_mac.mm
@@ -155,8 +155,24 @@ 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;
+    int64_t pointDeltaX = CGEventGetIntegerValueField(
+          [event CGEvent], kCGScrollWheelEventPointDeltaAxis2);
+    int64_t pointDeltaY = CGEventGetIntegerValueField(
+          [event CGEvent], kCGScrollWheelEventPointDeltaAxis1);
+
+    // A rounding bug on Sierra means that single-tick wheel scrolls may be
+    // truncated to zero. Prefer kCGScrollWheelEventPointDeltaAxisXY in those
+    // cases. First assume the CGEventSource has the default value of 10, rather
+    // than querying CGEventSourceGetPixelsPerLine() each time.
+    const float kPixelsPerLine = 10.0;
+    if (deltaX == 0 && pointDeltaX != 0)
+      deltaX = pointDeltaX * kPixelsPerLine;
+    if (deltaY == 0 && pointDeltaY != 0)
+      deltaY = pointDeltaY * kPixelsPerLine;
+
+    return gfx::Vector2d(deltaX, deltaY);
   }
 }