Directly Setting Child By Index Doesn't Work Correctly?

[AristurtleDev]

Description

I'm working on some extension methods in MonoGame.Extended for the MonoGame implementation of Gum. The Button class by default uses a ColoredRectangleRuntime as for the background, so I wanted to make an extension method to easily change it without the user having to create a full custom control. However, when changing the child element from a ColorRectangleRuntime to a NineSliceRuntime by setting it in the index directly, it does not render properly.

Reproduction

To recreate this, you can use the following extension method

public static NineSliceRuntime UseNinePatch(this Button button, Texture2D texture)
{
    if (button.Visual.GetChildByName("ButtonBackground") is IRenderableIpso child)
    {
        NineSliceRuntime nineSlice = new NineSliceRuntime();
        nineSlice.Width = 0;
        nineSlice.Height = 0;
        nineSlice.WidthUnits = DimensionUnitType.RelativeToContainer;
        nineSlice.HeightUnits = DimensionUnitType.RelativeToContainer;
        nineSlice.Name = "ButtonBackground";
        nineSlice.SourceFile = texture;

        int index = button.Visual.Children.IndexOf(child);

        // Setting directly by index here causes the issue
        button.Visual.Children[index] = nineSlice;
        return nineSlice;
    }

    return null;
}

Workaround

Instead of setting the child directly by index as shown above, you can instead remove the child then insert the new element at the same index and it works as expected

public static NineSliceRuntime UseNinePatch(this Button button, Texture2D texture)
{
    if (button.Visual.GetChildByName("ButtonBackground") is IRenderableIpso child)
    {
        NineSliceRuntime nineSlice = new NineSliceRuntime();
        nineSlice.Width = 0;
        nineSlice.Height = 0;
        nineSlice.WidthUnits = DimensionUnitType.RelativeToContainer;
        nineSlice.HeightUnits = DimensionUnitType.RelativeToContainer;
        nineSlice.Name = "ButtonBackground";
        nineSlice.SourceFile = texture;

        int index = button.Visual.Children.IndexOf(child);

        // Remove the old child and then insert the new one at the same index works
        button.Visual.Children.RemoveAt(index);
        button.Visual.Children.Insert(index, nineSlice);
        return nineSlice;
    }

    return null;
}

[vchelaru]

Fixed by f8688ea