拖离屏幕时出现图形错误

Question

I have wrote this code that will create a rounded corner border around a panel.

public void DrawRoundRect(Graphics g, Pen p, float X, float Y, float width, float height, float radius)
{
    GraphicsPath gp = new GraphicsPath();
    //Upper-right arc:
    gp.AddArc(X + width - (radius * 2), Y, radius * 2, radius * 2, 270, 90);
    //Lower-right arc:
    gp.AddArc(X + width - (radius * 2), Y + height - (radius * 2), radius * 2, radius * 2, 0, 90);
    //Lower-left arc:
    gp.AddArc(X, Y + height - (radius * 2), radius * 2, radius * 2, 90, 90);
    //Upper-left arc:
    gp.AddArc(X, Y, radius * 2, radius * 2, 180, 90);
    gp.CloseFigure();
    g.DrawPath(p, gp);
    gp.Dispose();
}

private void panel_Paint(object sender, PaintEventArgs e)
{
    Graphics v = e.Graphics;
    DrawRoundRect(v, Pens.White, e.ClipRectangle.Left, e.ClipRectangle.Top, e.ClipRectangle.Width - 1, e.ClipRectangle.Height - 1, 10);
    base.OnPaint(e);
}

It works fine, until it goes off screen and this happens:

Does anyone know what I've done wrong, or how to fix the issue?

Answer 1

You misunderstand what ClipRectangle means.

It doesn't tell you how big the panel is - it tells you which part of the panel to redraw. As you move the form off-screen, it needs to redraw the parts that changed monitors - but not the ones that remain on the original one. So instead of redrawing the whole panel (when ClipRectangle is the whole area of the panel, and your code works), it tells you to only redraw a part of it - and ClipRectangle is much smaller than the panel you're trying to draw.

ClipRectangle is really an optimization feature. If you don't care about that, you can pretty much ignore it completely - just use 0, 0, panel.Width, panel.Height .

Answer 2

You have two problems. First is relying on e.ClipRectangle, that is not the size of the panel. You need to use panel.ClientRectangle instead. That's what causes the smearing when you drag it off the screen and back.

You haven't found the next one yet, it is much more subtle. Panel was designed to be a container control and it optimizes its painting. Redrawing only the parts that get revealed when it is resized. Pretty important, makes resizing a window more bearable. That however will not work well for details like this, the rounded corners will not get properly repainted. Looks like a smearing effect as well, less pronounced than what you have now. It will happen whenever you resize the panel, typically with the Dock or Anchor property.

Proper way to do that is to derive your own class from Panel, set the ResizeRedraw property to true in the constructor. You typically almost always also want to set DoubleBuffered to true to suppress the visible flicker you now have. Or use the panel's Resize event, call panel.Invalidate().