Currently buttons fields has a few bits in the modifier field in WebInputEvent. This property belongs to WebPointerProperties and we can add its own field to the clarity.
Also currently we do not set this property for touch event path and if pen events come from that path we will not have that information to expose it to the web.
 

Currently buttons attribute has a few bits in the modifier field in WebInputEvent. This property belongs to WebPointerProperties and we can add its own field for the clarity.
Also currently we do not set this property for touch event path and if pen events come from that path we will not have that information to expose it to the web.
 

Currently buttons attribute has a few bits in the modifier field in WebInputEvent. This property belongs to WebPointerProperties and we can add its own field for the clarity.
Also currently we do not set this property as well as button for touch event path from the platform APIs and if pen events come from that path we will not have that information to expose it to the web.
 

Currently we do not set buttons and button fields for touch event path from the platform APIs and if pen events come from that path we will not have that information to expose it to the web.
On the Blink side the best approach would be to extract buttons from WebInputEvent modifiers and just keep it as a plain buttons field similar to what we did for button for clarity.