CollectionView OnCollectionViewScrolled Calls and parameters are inconsistent or incorrect

[david-maw]

Description

OnCollectionViewScrolled has a parameter that includes FirstVisibleItemIndex and LastVisibleItemIndex. It is not initially called in Windows but when it eventually is the values seem to be correct. On Android it is called initially, but the values are not correct (usually they are off by 1)

Steps to Reproduce

Forst clone the repository then build and run the app on Windows, you should see this:

Now build and run it on Android and you should see this:

Note that the "parameter" information indicates "Item 0 to Item 2", this is derived from a call on OnCollectionViewScrolled and there are two things to note:

1. The same call was not made on Windows (the information was blank)
2. The LastVisibleItemIndex value is 2 but the last visible item is actually at index 1.

Now check the "long list" box at the bottom of the page and the CollectionView will be populated with a longer list. Scroll it up one item and you'll see this:

3. On the screen you can see item 1 through most of item 7 but the call on OnCollectionViewScrolled indicated "Item 2 to Item 8"

4. Finally, go back to Windows and try the long list, note that SnapPointsType="Mandatory" seems to have no effect.

Link to public reproduction project repository

https://github.com/david-maw/OnCollectionViewScrolled.git

Version with bug

9.0.0-rc.2.24503.2

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

Unknown/Other

Affected platforms

Android, Windows, I was not able test on other platforms

Affected platform versions

Android 11 Windows 11

Did you find any workaround?

No, it fails both in .NET 8 and 9.

Relevant log output

No response

[jaosnz-rep]

I can repro this issue at Windows & Android platform on the latest 17.12.0 preview 5(9.0.0-rc.1.24453.9 & 9.0.0-rc.2.24503.2 & 8.0.82).