The one where I had to write my own ToolstripItemRenderer to get around a 'closed won't fix' bug in Windows Forms

Breakdown of time spent adding a new feature in NimbleSET:

So the bug in the framework is that if a toolbarstrip has large icons (e.g. ToolStrip.ImageScalingSize = new Size(32, 32);), then 'Checked' items won't render correctly in a child dropdown list of that toolbarstrip. Obscure? Yep.

checked items checkmark renders misaligned

(See the clipped checkmark next to the word 'Natural' that is rendered in the wrong place? That's why shipping software is hard.)

It's obscure enough that it got 0 upvotes and 0 repros on Connect. The workaround provided there wasn't enough to solve it properly. (Incidentally when I visit that page now I get a 400 error, but when i visit from an incognito browser it works... so I suspect I'm sending like a billion old cookies or something...)

What I needed to do was write my own ToolStripProfessionalRenderer, and override the OnRenderItemCheck method.

If you want to create your own renderer, avoid the advice that says to inherit from ToolStripRenderer, you want to inherit from ToolStripProfessionalRenderer instead, like so:

public class CustomToolStripRenderer : System.Windows.Forms.ToolStripProfessionalRenderer
{
}

And be sure to tell your control that you want to use your custom renderer, like this:

mainToolStrip.Renderer = new CustomToolStripRenderer();

Then within the custom renderer, you can override whichever method you're unhappy with.

Here's the code I wrote to draw a little blue box, and a custom tick, that should be a pixel-perfect representation of the intended one. I just know there are going to be edge cases (e.g. custom DPI) that will break this.

protected override void OnRenderItemCheck(ToolStripItemImageRenderEventArgs e)
{
    var g = e.Graphics;
    var fillColor = (e.Item.Selected ? this.ColorTable.CheckSelectedBackground : this.ColorTable.CheckBackground);
    g.FillRectangle(new SolidBrush(fillColor), new Rectangle(3, 1, 19, 19));
    g.DrawRectangle(new Pen(new SolidBrush(this.ColorTable.ButtonCheckedHighlightBorder), 1), new Rectangle(3, 1, 19, 19));
    // Draw the check mark. (two lines)
    g.DrawLines(System.Drawing.Pens.Black, new Point[] { new Point(10, 9), new Point(12, 11), new Point(16, 7) });
    g.DrawLines(System.Drawing.Pens.Black, new Point[] { new Point(10, 10), new Point(12, 12), new Point(16, 8) });
    // base.OnRenderItemCheck(e); <-- **don't** let the base do its own rendering.
}

But it got the job done, and allowed me to use the dormant GDI+ neurons I developed long ago.

checked items render correctly

For extra credit... make it work in high DPI scenarios. Oh, and support dark mode, or custom themes.

For high DPI scenarios, the critical piece of logic I use is this:

var height = (int)(19 * DisplayScaleFactor.Height);

And then abstract some more things away. Instead of drawing the lines twice, I have a helper function. I'll just give you the codes....

    // Custom rendered to overcome bug in WindowsForms
    protected override void OnRenderItemCheck(ToolStripItemImageRenderEventArgs e)
    {
        var g = e.Graphics;
        var width = 19;
        var height = (int)(19 * DisplayScaleFactor.Height);
        var offset = 6;
        // Determine fillcolor of rectangle around check mark
        var fillColor = (e.Item.Selected ? this.ColorTable.CheckSelectedBackground : this.ColorTable.CheckBackground);
        var rectangle = new Rectangle(2, 0, width, height);

        using (var fillBrush = new SolidBrush(fillColor))
        {
            g.FillRectangle(fillBrush, rectangle);
        }

        using (var highlightBrush = new SolidBrush(this.ColorTable.ButtonCheckedHighlightBorder))
        using (var highlightPen = new Pen(highlightBrush, 1))
        {
            g.DrawRectangle(highlightPen, rectangle);
        }

        DrawCheckMark(g, 0 + offset);
        DrawCheckMark(g, 1 + offset);

        // base.OnRenderItemCheck(e); <-- don't do the underlying render.
    }

    private void DrawCheckMark(Graphics g, int offset)
    {
        g.DrawLines(Pens.Black, new Point[] {
                new Point(9, 9 + offset),
                    new Point(11, 11 + offset),
                        new Point(15, 7 + offset) });
    }

(This isn't very cool... I just make the checkmark centred, but no bigger... so with a sufficiently dense DPI it becomes a very small checkmark.)