[Bug] FirstVisibleItemIndex is wrong when scrolling CollectionView on iOS

[vikern22]

Description

When listening to the Scrolled-event on a CollectionView using LinearItemsLayout(ItemSpacing="10", Orientation="Vertical", SnapPointsAlignment="Start", SnapPointsType="Mandatory"), the ItemsViewScrolledEventArgs.FirstVisibleItemIndex returns an index 1 number lower than the actual value on the iOS platform. Seems to work fine on the Android platform.

Steps to Reproduce

1. Run the attached sample
2. Scroll the CollectionView

Expected Behavior

The Label below the CollectionView should display the index of the item aligned at the top.

Actual Behavior

The Label below the CollectionView displays 1 number lower than the expected value.

Basic Information

• Version with issue: 4.5.0.396
• Last known good version:
• IDE: VS 2019 Enterprise
• Platform Target Frameworks:
  • iOS: 13.3
  • Android: 10
• Nuget Packages: NETStandard.Library, Xamarin.Essentials, Xamarin.Forms
• Affected Devices: iPhone 11 Emulator, iPhone 7

Reproduction Link

https://drive.google.com/open?id=1Ci6NjRY9KvE3RbYSDZQ4f0pV1IsGY_LN

[jfversluis]

Adding the reproduction here on GitHub as well so we won't lose it.

Confirmed this is happening with the reproduction. I have played around with the ItemSpacing, margins, snapping etc. but that doesn't seem to matter. It's not until item 1 is scrolled halfway off the top of the screen before the FirstVisibleItemIndex goes to 1, while that value should be 1 as soon as item 0 was off screen.

[InquisitorJax]

This is pretty important for us to get code scrolling to work... anyone know of a work-around to get the first visible item index?

[hartez]

Repro9989.zip

[znelson32]

Really wish this wasn't an issue

[InquisitorJax]

It's a little ugly, but here's a work-around for iOS using a custom renderer
CollectionViewWorkAround.zip

[chaoyebugao]

With XF 5.0.0.2012, problem still exists.

[jlinker7]

Has anyone else found a viable work around for this? Using InquisitorJax's solution doesn't seem to work for me (The ScrollTo method no longer works). Have a horizontal list the user can scroll, or tap an arrow to move right and left one item at a time, this bug is keeping it from working well.