The OP has confirmed in the comments that disabling the Win10 mouse setting "Scroll inactive windows when I hover over them" resolves the issue, however I believe that this can also be handled by preventing the MouseWheel event from bubbling up to the containing Panel control. Asking users to change their preferred settings to make your code function is never a desirable situation.
The following code demonstrates preventing this event bubbling. Just create a new Winform project and replace the Form1 code with this. The code creates a TextBox, and a PictureBox contained in a Panel. The purpose of the TextBox is to just to show its loss of focus when you click on the PictureBox. For Win7, click on the PictureBox to activate it and then use the mousewheel to increase/decrease the PictureBox size.
public partial class Form1 : Form
{
PictureBox pictureBox1;
Panel panel1;
public Form1()
{
InitializeComponent();
Size = new Size(500, 500);
Controls.Add(new TextBox() { TabIndex = 0, Location = new Point(350, 5)});
panel1 = new Panel() {Size = new Size(300, 300), Location = new Point(5, 5), BorderStyle = BorderStyle.FixedSingle,Parent = this, AutoScroll = true};
pictureBox1 = new PictureBox() {Size = new Size(200, 200) , Location = new Point(5,5), BorderStyle = BorderStyle.FixedSingle, Parent = panel1};
pictureBox1.Click += pictureBox1_Click;
pictureBox1.MouseWheel += pictureBox1_MouseWheel;
panel1.MouseWheel += panel1_MouseWheel;
}
private void pictureBox1_Click(object sender, EventArgs e)
{
// On Win10 with "Scroll inactive windows when I hover over them" turned on,
// this would not be needed for pictureBox1 to receive MouseWheel events
pictureBox1.Select(); // activate the control
// this makes pictureBox1 the form's ActiveControl
// you could also use:
// this.ActiveControl = pictureBox1;
}
private void pictureBox1_MouseWheel(object sender, MouseEventArgs e)
{
Rectangle r = pictureBox1.Bounds;
int sizeStep = Math.Sign(e.Delta) * 10;
r.Inflate(sizeStep, sizeStep);
r.Location = pictureBox1.Location;
pictureBox1.Bounds = r;
// e is an instance of HandledMouseEventArgs
HandledMouseEventArgs hme = (HandledMouseEventArgs)e;
// setting to true prevents the bubbling of the event to the containing control (panel1)
hme.Handled = true;
// comment out the above line to observe panel1_MouseWheel
// being called
}
private void panel1_MouseWheel(object sender, MouseEventArgs e)
{
System.Diagnostics.Debug.Print("bubbled wheel event");
}
}
reference: HandledMouseEventArgs Class
