Winforms groupbox with colored border

Question

I have used the following code to create a groupbox with colored borders:

Public Class BorderGroupBox
    Inherits GroupBox

    Private _borderColor As Color
    Private _borderWidth As Integer
    Private _borderStyle As ButtonBorderStyle

    ...

    Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)
        Dim tSize As Size = TextRenderer.MeasureText(Me.Text, Me.Font)
        Dim borderRect As Rectangle = e.ClipRectangle
        borderRect.Y = CInt((borderRect.Y + (tSize.Height / 2)))
        borderRect.Height = CInt((borderRect.Height - (tSize.Height / 2)))
        ControlPaint.DrawBorder(e.Graphics, borderRect, _borderColor, _borderWidth, _borderStyle, BorderColor, _borderWidth, _borderStyle, BorderColor, _borderWidth, _borderStyle, BorderColor, _borderWidth, _borderStyle)
        Dim textRect As Rectangle = e.ClipRectangle
        textRect.X = (textRect.X + 6)
        textRect.Width = tSize.Width + 6
        textRect.Height = tSize.Height
        e.Graphics.FillRectangle(New SolidBrush(Me.BackColor), textRect)
        e.Graphics.DrawString(Me.Text, Me.Font, New SolidBrush(Me.ForeColor), textRect)
    End Sub
End Class

The problem is, it is placed inside a scrollable container, and if it is scrolled the border isn't redrawn correctly:

Answer 1

You can get it to misbehave a lot worse than that:

This goes wrong because of your code using e.ClipRectangle. Note that it appears twice in your snippet. That variable does not give you the border rectangle. It tells you how much of your client area needs to be re-drawn. It is an optimization opportunity, you can draw less by omitting the parts of the client area that don't need to be refreshed.

It is usually the same size as the display rectangle, which is why it looked like it worked just fine. But not when you put it inside a scrollable container, Windows optimizes scrolls by blitting the parts of the client area that simply can be moved. And then generates a paint for the parts that are revealed by the scroll. With a small e.ClipRectangle. You can see that in the screenshot, note the small rectangles.

Replace e.ClipRectangle with Me.DisplayRectangle.