MouseEvent.button should be 0 for mousemove events Spec: https://www.w3.org/TR/uievents/#dom-mouseevent-button > The value of button is not updated for events not caused by the > depression/release of a mouse button. In these scenarios, take > care not to interpret the value 0 as the left button, but rather > as the un-initialized value. test case 1: attachment 377952 [details] test case 2: https://jsfiddle.net/fujihiro/vy7hkpLw/10/embedded/result Firefox and Chrome set it 0 for mousemove events.
This seems to work in Safari. Is this a Windows port bug report?
This is a test procedure: 1. Open attachment 377952 [details] 2. Hold the mouse middle button pressing 3. Move the mouse button should be 0, but 1. I tested with Safari Technology Preview 91 and Safari 12.1.2.
<rdar://problem/55180368>