Pressure should not be set when the pointer device doesn't support pressure

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 387 matching lines @@
   // and mouses share the same cursor, so we will set their pointerType as
   // Unknown for now.
   if (type == NSMouseExited || type == NSMouseEntered) {
     result.pointerType = blink::WebPointerProperties::PointerType::Unknown;
     return result;
   }
 
   // For other mouse events and touchpad events, the pointer type is mouse.
   // For all other tablet events, the pointer type will be just pen.
   NSEventSubtype subtype = [event subtype];
-  if (subtype == NSTabletPointEventSubtype ||
-      subtype == NSTabletProximityEventSubtype) {
-    result.pointerType = blink::WebPointerProperties::PointerType::Pen;
-  } else {
+  if (subtype != NSTabletPointEventSubtype &&
+      subtype != NSTabletProximityEventSubtype) {
     result.pointerType = blink::WebPointerProperties::PointerType::Mouse;
+    return result;

[mustaq, 2016/09/21 19:39:36]

- Could you please confirm that uninitialized result.force gets a NaN value here
as opposed to 0 or random? For result.tiltX/Y, uninitialized values are always
0, right?

- You are now setting the value by relying on pointerType. Is there a way to
query the device if pressure is supported or not? If not sure, please add a
TODO.

[lanwei, 2016/09/21 20:13:42]

Yes, default values of force is NaN, tiltx, y are 0,
https://codesearch.chromium.org/chromium/src/third_party/WebKit/public/platfo...

There is no way to query a specific property, in their spec, they said we can
use the subtype to decide if the event is 'pointer events', the event that a
tablet device generates after a stylus has entered proximity of the tablet, the
pressure, rotation, tilt are defined under this event.

   }
+
+  // Set stylus properties for events with a subtype of
+  // NSTabletPointEventSubtype.
+  result.pointerType = blink::WebPointerProperties::PointerType::Pen;
   result.id = [event deviceID];
-  result.force = [event pressure];
-  NSPoint tilt = [event tilt];
-  result.tiltX = lround(tilt.x * 90);
-  result.tiltY = lround(tilt.y * 90);
+  if (subtype == NSTabletPointEventSubtype) {
+    result.force = [event pressure];
+    NSPoint tilt = [event tilt];
+    result.tiltX = lround(tilt.x * 90);
+    result.tiltY = lround(tilt.y * 90);
+  }
   return result;
 }
 
 // WebMouseWheelEvent ---------------------------------------------------------
 
 blink::WebMouseWheelEvent WebMouseWheelEventBuilder::Build(
     NSEvent* event,
     NSView* view) {
   blink::WebMouseWheelEvent result;
 
@@ skipping 181 matching lines @@
       break;
     default:
       NOTIMPLEMENTED();
       result.type = blink::WebInputEvent::Undefined;
   }
 
   return result;
 }
 
 }  // namespace content