Looks good overall!

I did notice an off by one error that's easier to see when you're zoomed in on a really small image
It's probably because RectangleI.Right is the x coordinate of the last pixel that's in the rectangle , so computing Width as Right - Left is off by one

You are right @cameronwhite, although the selection bounds are a RectangleD. For clarity I changed it the ends of the selection to bounds.Left + bounds.Width and bounds.Top + bounds.Height and made them nullable (to indicate the absence/presence of a selection).

As for the selection color, I think the one I chose is more similar to Paint.NET's. Not that Pinta has to copy its features, but it's worth noting for context.

The color you suggested is on commit bccde34 and it looks like this:

On a different, but related note, looking through the code, I found that the ToInt method in RectangleD is like this:

public readonly record struct RectangleD (
	double X,
	double Y,
	double Width,
	double Height)
{
	// ...
	
	public readonly RectangleI ToInt ()
		=> new (
			(int) Math.Floor (X),
			(int) Math.Floor (Y),
			(int) Math.Ceiling (Width),
			(int) Math.Ceiling (Height));
	
	// ...
}

I think this could be unclear or misleading. Maybe we should add an enum-typed parameter that specifies exactly how it should be rounded?

Thanks! The original color looks better, but maybe we can make it a more subtle (e.g. a bit more transparent perhaps?)
Right now I think it stands out a bit too much versus the selected widgets in the panels , for example

We could actually use https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/method.StyleManager.get_accent_color_rgba.html to get the exact same color, but I'm not sure if we want that since the accent color can vary based on system settings (https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/css-variables.html#accent-colors), and I think this should stay blue to match the selection. Using the same blue color could be worth trying though

@cameronwhite now the color is more similar to Adwaita's accent color. I normalized the channels to a range of 0-1, rounded it to two decimal places, and set an alpha of 0.25.

Thanks! That colour looks good to me 👍