UWP - Middle Mouse Wheel Button Click Event Handler

With these it helps You can get the middle-mouse button with GetAsyncKeyState(4). But you'll never get mouse scroll messages with this approach, there isn't any way to ask for the scroll button position. You must rely on the Window message that tells you that the mouse was scrolled, WM_MOUSEWHEEL.
The requirement that a recording is made even if your own window doesn't have the focus requires a fundamentally different approach, you need to use a low-level hook. A hook set by SetWindowsHookEx(), it will call a method in your program when a mouse event occurs.

I hope this helps . From its name, the mousePressEvent is responsible for mouse clicks while the wheelEvent is for scrolling solely. The wheelEvent will not catch the wheel button click. It is how the Qt's API is designed when it comes to mouse events processing.
In order to separate which mouse button is pressed (right, wheel or left), use button property of QMouseEvent.

void GLWidget::mousePressEvent(QMouseEvent *event) // redefine the mouse event
{
switch( event->button() ) {
case Qt::LeftButton:
    // do stuff for left button pressed
    break;
case Qt::MiddleButton:
    // do stuff for wheel button pressed
    break;
// ...
}
}

To fix this issue You need to handle more than one event. You need to capture the MouseWheel event and the MouseDown/MouseUP events.

Private mouseDown As Boolean = False
Private Sub Form1_MouseWheel(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseWheel
    If e.Delta > 0 And mouseDown Then
        Console.WriteLine("Scrolling with button down")
    Else
        Console.WriteLine("Button Not down whilst scrolling")
    End If
End Sub

Private Sub Form1_MouseDown(sender As Object, e As MouseEventArgs) Handles MyBase.MouseDown
    If e.Button = MouseButtons.Middle Then
        mouseDown = True
    End If
End Sub

Private Sub Form1_MouseUp(sender As Object, e As MouseEventArgs) Handles MyBase.MouseUp
    If e.Button = MouseButtons.Middle Then mouseDown = False
End Sub

fixed the issue. Will look into that further OpenSeadragon doesn't have a flag for that, but you can easily build it using the MouseTracker. Here's an example (coded from memory and not tested, but it should give you the idea).

var drag;

var mouseTracker = new OpenSeadragon.MouseTracker({
  element: viewer.container, 
  nonPrimaryPressHandler: function(event) {
    if (event.button === 1) { // Middle
      drag = { 
        lastPos: event.position.clone()
      };
    }
  },
  moveHandler: function(event) {
    if (drag) {
      var deltaPixels = drag.lastPos.minus(event.position);
      var deltaPoints = viewer.viewport.deltaPointsFromPixels(deltaPixels);
      viewer.viewport.panBy(deltaPoints);
      drag.lastPos = event.position.clone();
    }
  },
  nonPrimaryReleaseHandler: function(event) {
    if (event.button === 1) {
      drag = null;
    }
  }
});

Hope that helps For secondary mouse buttons, you can handle the auxclick event. In the following example, the click event is triggered by the primary mouse button, and the auxclick event is triggered by the other mouse buttons.

<a (click)="onClick($event)" (auxclick)="onAuxClick($event)">...</a>

onClick(event: MouseEvent) {
  console.log("click - button", event.button, event.which);
}

onAuxClick(event: MouseEvent) {
  console.log("auxclick - button", event.button, event.which);
}