[iOS] CollectionView fails to render items when frame grows from zero post-mount (regression of #30953 in .NET 10)

CollectionViewHandler2 (the default iOS handler in .NET 10) fails to render items when the underlying UICollectionView
is mounted with a zero-size frame and then later grows. This is the exact symptom reported in #30953
(https://github.com/dotnet/maui/issues/30953), which was marked closed via a fix that shipped to .NET 9 SR10 (PR
#30978 (https://github.com/dotnet/maui/pull/30978)). The fix was not forward-ported to .NET 10 — the regression is
back on MAUI 10.0.60.

Tracing the .NET 10 source (src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs):

- LoadView creates the UICollectionView with CGRect.Empty.
- GetDesiredSize has a force-layout fallback for pre-mount measurement, but only fires when collectionView.Window ==
null — once the view is mounted with a zero frame, this path is bypassed forever.
- ViewWillLayoutSubviews re-invalidates only when NeedsCellLayout is explicitly set or ContentSize changes. Neither
happens, because items were never realized to begin with.

The result: items are never realized, even though the parent layout has grown to give the CollectionView a non-zero
frame.

Steps to Reproduce

Minimal pattern:

1. Create a Grid with a RowDefinition whose Height is bound to a property starting at 0.
2. Place a CollectionView in that row, with HeightRequest bound to the same (or another zero-starting) property.
3. On a button tap, update the property to a non-zero value.

Expected: items render once the row grows.
Actual: the CollectionView is empty on iOS in .NET 10 (Handler2). Same code works on .NET 9 (legacy handler) and on
iOS in .NET 10 if the legacy handler is registered explicitly.

The pattern is the same one in #30953's repro project:
https://github.com/telerik/ms-samples/tree/main/Maui/CollectionViewItemsSourceNet10Issue. Reproduces identically on
MAUI 10.0.60.

Link to public reproduction project repository

Same repro as #30953: https://github.com/telerik/ms-samples/tree/main/Maui/CollectionViewItemsSourceNet10Issue (update
package versions to 10.0.60 to reproduce on .NET 10).

Version with bug

10.0.60 (also reproduced on 10.0.41)

Is this a regression from previous behavior?

Yes

Last version that worked well

9.0.90 SR9 (per #30953, and confirmed against the legacy CollectionViewHandler on .NET 10 — fix from PR #30978
evidently didn't forward-port to .NET 10).

Affected platforms

iOS, Mac Catalyst (per #30953)

Affected platform versions

iOS 26.4

Did you find any workaround?

Yes — register the legacy CollectionViewHandler for the affected CollectionView (matches the workaround in the
original #30953):

#if IOS
builder.ConfigureMauiHandlers(handlers =>
{
handlers.AddHandler();
});
#endif

To scope the opt-out to a single control instead of every CollectionView in the app, register against a thin subclass:

public class MyCollectionView : CollectionView { }

handlers.AddHandler();

Both restore correct rendering. Handler2 remains active for all other CollectionView instances when scoped via
subclass.

Relevant log output

No exceptions or warnings — the CollectionView simply renders empty

Start in src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs and trace LoadView, GetDesiredSize, and ViewWillLayoutSubviews. Compare the missing .NET 10 behavior with the fix described in PR #30978, then run the CollectionViewItemsSourceNet10Issue reproduction on MAUI 10.0.60. Done means items render after the CollectionView grows from a zero-size frame on iOS and Mac Catalyst without requiring the legacy handler.

Written by the indexing model from the issue text.