Fix MouseWheelEventArgs.Delta from static to an instance readonly field

[h3xds1nz]

Description

Similarly as in #7910, the MouseWheelEventArgs.Delta is a static field, meaning a creation of a new event changes the Delta value for all previously created events. This is clearly a bug, the field shall not be static but an instanced readonly as the event should describe the state at the exact point of time it was generated.

Customer Impact

Creation of a new MouseWheelEventArgs will not influence the value of Delta for previous instances.

Regression

No.

Testing

Local build.

Risk

Low.

Microsoft Reviewers: Open in CodeFlow

[miloush]

the event should describe the state at the exact point of time it was generated.

That is not such a clear cut, since as discussed in a few other issues here, mouse events/args tend to return the current state of the device rather than the state at the time the args were created. See the buttons state properties, coming from MouseDevice.GetButtonStateFromSystem or even the GetPosition, which reports the last received mouse position for any instance of the args. I can see how a similar behavior could have been the intent here (and in that light, the #7910 fix is questionable).

That said, wheel delta is not a state, it's an event, and it seems reasonable that you should be able to accumulate them, and accumulating with the current implementation gives you potentially incorrect results.

[h3xds1nz]

Totally forgot; just wanted to say I fully agree with the statement @miloush, was just trying too hard to sell it.