[iOS/Mac/Windows] CollectionView ItemsViewScrolledEventArgs are incorrect when IsGrouped = true

[matmork]

Description

Basically this from Xamarin.Forms: xamarin/Xamarin.Forms#10389

Copy/Paste:
When a CollectionView's IsGrouped property is set to true, the values of FirstVisibleItemIndex, CenterItemIndex, and LastVisibleItemIndex specified in the ItemsViewScrolledEventArgs passed to the Scrolled event handler are incorrect. They appear to be unable to reflect the indicies of any items beyond the first group.

NET8-Android.mov

NET8-Ios.mov

Steps to Reproduce

1. Run the iOS app.
2. Scroll the list.
3. See Label with text from FirstVisibleItemIndex not updating

Link to public reproduction project repository

https://github.com/Matmork/CollectionViewScroll

Version with bug

8.0.0-rc.1.9171

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

Unknown/Other

Affected platforms

iOS

Affected platform versions

No response

Did you find any workaround?

No response

Relevant log output

No response

[Petarcarls997]

09425100

[XamlTest]

Verified this on Visual Studio Enterprise 17.9.0 Preview 1(8.0.3). Repro on iOS 17.0 and MacCatalyst, not repro on Windows 11 and Android 14.0-API34 with below Project:

CollectionViewScroll.zip

[marcustindab]

@samhouts
This also seems to be faulty on Android. Tested on a Samsung S21 device while running Visual Studio 2022.

[slouge]

On Android i found we need to count +1 for each group header and add also 1 if you have a group footer.
On Windows it's not the same thing, you just need to count +1 ...
On iOS completely broken, FirstVisibleIndex is always 0