Mac: Support Sierra's broken MouseWheel events.

ui/events/cocoa/events_mac.mm

 // Copyright 2014 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.
 
 #include "ui/events/event_utils.h"
 
 #include <Cocoa/Cocoa.h>
 #include <stdint.h>
 
 #include "base/logging.h"
@@ skipping 137 matching lines @@
     // CGEventGetIntegerValueField(kCGScrollWheelEventPointDeltaAxis{1|2}).
     return gfx::Vector2d([event scrollingDeltaX], [event scrollingDeltaY]);
   } else {
     // Empirically, a value of 0.1 is typical for one mousewheel click. Positive
     // values when scrolling up or to the left. Scrolling quickly results in a
     // higher delta per click, up to about 15.0. (Quartz documentation suggests
     // +/-10).
     // 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);
   }
 }
 
 base::NativeEvent CopyNativeEvent(const base::NativeEvent& event) {
   return [event copy];
 }
 
 void ReleaseCopiedNativeEvent(const base::NativeEvent& event) {
   [event release];
 }
@@ skipping 130 matching lines @@
   uint16_t return_value;
   [text getCharacters:&return_value];
   return return_value;
 }
 
 bool IsCharFromNative(const base::NativeEvent& native_event) {
   return false;
 }
 
 }  // namespace ui