Why is the "Paint" event not raised by buttons that have the FlatStyle property set to "System"? Using Flat, Standard and Popup everything works fine. I searched for similar questions and read the official documentation, but I didn't find anything about this particular problem.

Here is a test code to check what I mean:

C# code

private void Form1_Load(object sender, EventArgs e)
{
    button1.FlatStyle = FlatStyle.Standard;
    button2.FlatStyle = FlatStyle.System;
    button1.Paint += Button_Paint;
    button2.Paint += Button_Paint;
}

private void Button_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.DrawLine(Pens.Black, 0, 0, 100, 50);
}

VB.NET code

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Button1.FlatStyle = FlatStyle.Standard
    Button2.FlatStyle = FlatStyle.System
End Sub

Private Sub Button1_Paint(sender As Object, e As PaintEventArgs) Handles Button1.Paint, Button2.Paint
    e.Graphics.DrawLine(Pens.Black, 0, 0, 100, 50)
End Sub

Output