[iOS] RecalculateSpanPositions causing crash

[artemvalieiev]

Description

Using formatted text with custom font and mixed spans( see Steps to reproduce example of FormattedText that triggers exception) is causing crash.
I am happy to create a PR for this fix with my calculation (see workaround), but I need 1-2 days to get know UI tests.
8.0.3 was working because there wasn't this method. 8.0.21/ 8.0.40 contains this issue

Steps to Reproduce

1. Create formatted string

public FormattedString CreateFormattedString()
{
    string upperText =
        "A culture of continuous learning isn't something you can ignore. 76% of employees are more inclined to stay when their workplace offers learning and development 🤔 Here's a few ways you can foster this:🧠 Create channels to encourage knowledge sharing between team members👏 Provide learning opportunities like job shadowing and rotation for hands-on experiences💬 Integrate routine feedback as a celebrated part of the workflow💻 Encourage your teams to dive into digital learning and live events Take a look at some more methods you can adopt from this recent article in Forbes 👇 🔗 ";

    var tapLinkGestureRecognizer = new TapGestureRecognizer
    {
        CommandParameter =
            "https://www.forbes.com/sites/forbeshumanresourcescouncil/2024/04/11/3-actionable-steps-for-managers-to-cultivate-a-learning-culture/",
        Command = new Command<string>(async (url) =>
        {
            Debug.WriteLine(url);
            await Launcher.OpenAsync(new Uri(url));
        })
    };

    return new FormattedString
    {
        Spans =
        {
            new Span { Text = upperText, FontFamily = "GilroyLight", FontSize = 14 },
            new Span
            {
                GestureRecognizers = { tapLinkGestureRecognizer },
                Text =
                    "https://www.forbes.com/sites/forbeshumanresourcescouncil/2024/04/11/3-actionable-steps-for-managers-to-cultivate-a-learning-culture/",
                FontFamily = "GilroyLight", FontSize = 14, TextColor = Colors.LightSkyBlue,
                TextDecorations = TextDecorations.Underline
            }
        }
    };
}

2. On empty page add Label with margin 16

<?xml version="1.0" encoding="utf-8"?>

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MauiApp1.MainPage">

    <Grid>
        <Label x:Name="CrashLabel" FontFamily="GilroyLight" FontSize="14" FormattedText="{Binding FormattedString}" Margin="16,0" />
    </Grid>

</ContentPage>

3. Assign FormattedText to CreateFormattedString() method result

protected override void OnAppearing()
{
    base.OnAppearing();
    this.CrashLabel.FormattedText = CreateFormattedString();
}

Link to public reproduction project repository

https://github.com/artemvalieiev/maui-ios-span-calc-crash

Version with bug

8.0.40 SR5

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

8.0.3 GA

Affected platforms

iOS

Affected platform versions

No response

Did you find any workaround?

Yes, we use custom span region calculation, and we had to use custom Label because RecalculateSpanPositions is located in ArrangeOverride.

public partial class EngageMauiLabel_iOS
{
    protected override Size ArrangeOverride(Rect bounds)
    {
        Frame = this.ComputeFrame(bounds);
        Handler?.PlatformArrange(Frame);
        if (Handler?.PlatformView is UILabel label)
                label.RecalculateSpanPositions(this);
        return Frame.Size;
    }
}

public static class EngageMauiLabelExtensions
{
    public static void RecalculateSpanPositions(this UILabel control, Label element)
    {
        if (element is null)
            return;

        if (element.TextType == TextType.Html)
            return;

        if (element?.FormattedText?.Spans is null
            || element.FormattedText.Spans.Count == 0)
            return;

        var finalSize = control.Bounds;

        if (finalSize.Width <= 0 || finalSize.Height <= 0)
            return;

        var inline = control.AttributedText;

        if (inline is null)
            return;

        UpdateSpanRegions(control, element.FormattedText.Spans.ToList());
    }


    public static void UpdateSpanRegions(UILabel label, List<Span> spans)
    {
        var attributedText = label.AttributedText;
        if ((attributedText?.Length ?? 0) == 0)
            return;

        using var textStorage = new NSTextStorage();
        using var layoutManager = new NSLayoutManager();
        using var textContainer = new NSTextContainer();

        textStorage.AddLayoutManager(layoutManager);
        layoutManager.AddTextContainer(textContainer);

        textContainer.LineFragmentPadding = 0;
        textContainer.Size = new(label.Bounds.Width, nfloat.MaxValue);

        textStorage.SetString(attributedText!);

        layoutManager.EnsureLayoutForTextContainer(textContainer);

        int currentIndex = 0;
        
        foreach (var span in spans)
        {
            if (string.IsNullOrEmpty(span.Text))
                continue;

            var spanRects = new List<CGRect>();
            var spanStartIndex = currentIndex;
            var spanEndIndex = currentIndex + span.Text.Length - 1;

            var startGlyphRange = layoutManager.GetGlyphRange(new NSRange(spanStartIndex, 1));
            var endGlyphRange = layoutManager.GetGlyphRange(new NSRange(spanEndIndex, 1));

            void EnumerateLineFragmentCallback(CGRect rect, CGRect usedRect, NSTextContainer textContainer,
                NSRange lineGlyphRange, out bool stop)
            {
                
                //Whole span is within the line and bigger than the line
                if(lineGlyphRange.Location >= startGlyphRange.Location &&
                   NSMaxRange(lineGlyphRange) <= endGlyphRange.Location)
                {
                    spanRects.Add(usedRect);
                }
                // Whole span is within the line and smaller than the line
                else if (lineGlyphRange.Location <= startGlyphRange.Location &&
                         endGlyphRange.Location <= NSMaxRange(lineGlyphRange))
                {
                    var spanBoundingRect = layoutManager!.GetBoundingRect(
                        new(startGlyphRange.Location, endGlyphRange.Location - startGlyphRange.Location + 1),
                        textContainer);
                    spanRects.Add(spanBoundingRect);
                }
                // Span starts on current line and ends on next lines
                else if (lineGlyphRange.Location <= startGlyphRange.Location &&
                         NSMaxRange(lineGlyphRange) <= endGlyphRange.Location)
                {
                    var spanBoundingRect = layoutManager!.GetBoundingRect(
                        new(startGlyphRange.Location, NSMaxRange(lineGlyphRange) - startGlyphRange.Location + 1),
                        textContainer);
                    spanRects.Add(spanBoundingRect);
                }
                // Span starts on previous lines and ends on current line
                else if (lineGlyphRange.Location >= startGlyphRange.Location &&
                         NSMaxRange(lineGlyphRange) >= endGlyphRange.Location)
                {
                    var spanBoundingRect = layoutManager!.GetBoundingRect(
                        new(lineGlyphRange.Location, endGlyphRange.Location - lineGlyphRange.Location + 1),
                        textContainer);
                    spanRects.Add(spanBoundingRect);
                }

                stop = false;
            }

            layoutManager.EnumerateLineFragments(
                new(startGlyphRange.Location, endGlyphRange.Location - startGlyphRange.Location + 1),
                EnumerateLineFragmentCallback);

            var inflate = span.GestureRecognizers.Count > 0 ? 4 : -4;
            var region = Region.FromRectangles(spanRects.Select(r => new Rect(r.X, r.Y, r.Width, r.Height)))
                .Inflate(inflate);
            (span as ISpatialElement).Region = region;
            
            currentIndex += span.Text.Length;
        }
    }

    public static nint NSMaxRange(NSRange range) => range.Location + range.Length;
}
```,

### Relevant log output

```shell
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
   at System.Collections.Generic.List`1[[System.Double, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].get_Item(Int32 index)
   at Microsoft.Maui.Controls.Platform.FormattedStringExtensions.CreateSpanRects(CGRect startRect, CGRect endRect, List`1 lineHeights, List`1 multilineRects, Boolean endsWithNewLine)
   at Microsoft.Maui.Controls.Platform.FormattedStringExtensions.GetMultilinedBounds(NSRange characterRange, NSLayoutManager layoutManager, NSTextContainer textContainer, CGRect startRect, CGRect endRect, List`1 lineHeights, Boolean endsWithNewLine)
   at Microsoft.Maui.Controls.Platform.FormattedStringExtensions.RecalculateSpanPositions(UILabel control, Label element)
   at Microsoft.Maui.Controls.Label.RecalculateSpanPositions()
   at Microsoft.Maui.Controls.Label.ArrangeOverride(Rect bounds)
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IView.Arrange(Rect bounds)
   at Microsoft.Maui.Layouts.GridLayoutManager.ArrangeChildren(Rect bounds)
   at Microsoft.Maui.Controls.Layout.CrossPlatformArrange(Rect bounds)
   at Microsoft.Maui.Platform.MauiView.CrossPlatformArrange(Rect bounds)
   at Microsoft.Maui.Platform.MauiView.LayoutSubviews()
   at UIKit.UIApplication.UIApplicationMain(Int32 argc, String[] argv, IntPtr principalClassName, IntPtr delegateClassName) in /Users/builder/azdo/_work/1/s/xamarin-macios/src/UIKit/UIApplication.cs:line 58

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