Mac: Support Sierra's broken MouseWheel events.

content/browser/renderer_host/input/web_input_event_builders_mac.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006-2009 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 533 matching lines @@
   DCHECK(cg_event);
 
   // Wheel ticks are supposed to be raw, unaccelerated values, one per physical
   // mouse wheel notch. The delta event is perfect for this (being a good
   // "specific edge case" as mentioned above). Trackpads, unfortunately, do
   // event chunking, and sending mousewheel events with 0 ticks causes some
   // websites to malfunction. Therefore, for all continuous input devices we use
   // the point delta data instead, since we cannot distinguish trackpad data
   // from data from any other continuous device.
 
+  int64_t pointDeltaX = CGEventGetIntegerValueField(
+        cg_event, kCGScrollWheelEventPointDeltaAxis2);
+  int64_t pointDeltaY = CGEventGetIntegerValueField(
+        cg_event, kCGScrollWheelEventPointDeltaAxis1);
+
   if (CGEventGetIntegerValueField(cg_event, kCGScrollWheelEventIsContinuous)) {
-    result.deltaX = CGEventGetIntegerValueField(
-        cg_event, kCGScrollWheelEventPointDeltaAxis2);
-    result.deltaY = CGEventGetIntegerValueField(
-        cg_event, kCGScrollWheelEventPointDeltaAxis1);
-    result.wheelTicksX = result.deltaX / ui::kScrollbarPixelsPerCocoaTick;
-    result.wheelTicksY = result.deltaY / ui::kScrollbarPixelsPerCocoaTick;
+    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 =
         CGEventGetIntegerValueField(cg_event, kCGScrollWheelEventDeltaAxis2);
   }
 
   result.timeStampSeconds = [event timestamp];
 
   result.phase = PhaseForEvent(event);
   result.momentumPhase = MomentumPhaseForEvent(event);
@@ skipping 40 matching lines @@
       break;
     default:
       NOTIMPLEMENTED();
       result.type = blink::WebInputEvent::Undefined;
   }
 
   return result;
 }
 
 }  // namespace content