TransformGroup.Children replacement leaks child transform PropertyChanged subscriptions #36033

Description

Description
TransformGroup subscribes to each child transform’s PropertyChanged event when the child is added to Children, but replacing the entire Children collection only unsubscribes from the old collection’s CollectionChanged event. It does not unsubscribe from the child transforms already in the old collection.

If a child transform is long-lived, such as a static/shared resource, singleton, or cached visual state, it can retain the abandoned TransformGroup. When that group was assigned to Path.RenderTransform, the retained group can also keep the old Path and its BindingContext alive.

Repro:
https://github.com/AdamEssenmacher/maui/tree/repro/transformgroup-children-leak/src/Controls/samples/TransformGroupLeakRepro

With the default repro settings on Mac Catalyst, the affected scenario retains:

Paths: 480/480
TransformGroups: 480/480
payload view models: 40/40
Payload retained by alive view models: 120.0 MB

The control and mitigation scenarios return to zero or near-zero retained objects. The mitigation removes the shared child transform from Children before replacing the collection, which points to the stale child PropertyChanged subscription as the retention root.

Steps to Reproduce

Repro project:
https://github.com/AdamEssenmacher/maui/tree/repro/transformgroup-children-leak/src/Controls/samples/TransformGroupLeakRepro

Steps

1. Clone/checkout the repro branch:

   git clone https://github.com/AdamEssenmacher/maui.git
   cd maui
   git checkout repro/transformgroup-children-leak

2. Build the MAUI build tasks:

   dotnet build Microsoft.Maui.BuildTasks.slnf

3. Run the Mac Catalyst repro:

   dotnet run --project src/Controls/samples/TransformGroupLeakRepro/TransformGroupLeakRepro.csproj -f net10.0-maccatalyst

4. Leave the default settings:
   Pages/run = 40, Paths/page = 12, Payload MB/page = 3, Dwell ms/page = 25.

5. Click Run control.
   Expected: Paths, TransformGroups, and payload view models are 0/... or near zero after full GC.

6. Click Run shared transform.
   Expected on an affected build:
   Paths: 480/480, TransformGroups: 480/480, payload view models: 40/40, retained payload about 120 MB.

7. Click Run mitigation.
   Expected: counts return near the control run, usually 0/480 paths, 0/480 groups, and 0/40 payloads.

Why this demonstrates the leak

The leaky scenario adds a long-lived shared ScaleTransform to each TransformGroup, then replaces TransformGroup.Children. The old child transform keeps a stale PropertyChanged subscription to the abandoned TransformGroup.

The mitigation scenario removes the child transform from Children before replacing the collection. That returns retention to baseline, showing the stale child-transform event subscription is the retention root.

Link to public reproduction project repository

https://github.com/AdamEssenmacher/maui/tree/repro/transformgroup-children-leak/src/Controls/samples/TransformGroupLeakRepro

Version with bug

10.0.70

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, macOS, I was not able test on other platforms

Affected platform versions

No response

Did you find any workaround?

No response

Relevant log output