Use weak events for GradientBrush gradient stop subscriptions

[wbv-profile]

Description of Change

GradientBrush subscribes to its GradientStops collection and to each GradientStop with plain CLR event handlers, and only unsubscribes when the GradientStops property is replaced. A GradientStopCollection (or an individual stop) that outlives the brush therefore keeps the brush — and everything it references — alive.

There are two distinct retention paths:

1. The collection. UpdateGradientStops did newCollection.CollectionChanged += OnGradientStopCollectionChanged. A long-lived collection, for example one declared in a ResourceDictionary and shared between brushes, roots every transient brush ever assigned it.

2. The stops, after Clear(). OnGradientStopCollectionChanged detached handlers only from e.OldItems. ObservableCollection.Clear() raises a Reset, which carries no OldItems, so the removed stops kept both their PropertyChanged subscription and their Parent back-reference to the brush.

This change routes both subscriptions through the existing weak-event primitives in src/Controls/src/Core/Internals/WeakEventProxy.cs (WeakNotifyCollectionChangedProxy / WeakNotifyPropertyChangedProxy), following the pattern already used by Border and Shape, and adds a finalizer that unsubscribes — as the proxy documentation requires.

Because a Reset reports no old items and arrives after the collection has already been emptied, the brush now tracks the stops it is subscribed to in a Dictionary<GradientStop, WeakNotifyPropertyChangedProxy>. That set is the only record of what still needs detaching on Clear(), and it also lets a stop that legitimately appears more than once in a collection be detached only when its last occurrence is gone.

Behaviour is otherwise unchanged: collection changes and stop property changes still raise InvalidateGradientBrushRequested, and stops attached to a brush still get their Parent set. Stops removed by Clear() now correctly have Parent reset to null, which previously leaked as a stale parent.

Note that GradientStop.Parent remains a strong reference by design, so the brush that currently owns a set of stops is still reachable from them; this change only stops discarded brushes from being retained.

Tests

src/Controls/tests/Core.UnitTests/GradientBrushTests.cs adds:

• SharedEmptyCollectionDoesNotRootBrush — isolates the CollectionChanged path (empty collection, so Parent cannot mask it).
• SharedPopulatedCollectionDoesNotRootDiscardedBrush — a second brush takes over Parent, leaving the first retained only by its subscriptions.
• RetainedStopDoesNotRootBrushAfterClear — the Reset/Clear() path.
• ClearDetachesStopParent and ClearStopsInvalidationFromRemovedStop — behavioural cover for the Reset fix.
• AttachedStopStillRaisesInvalidation and CollectionChangesStillRaiseInvalidation — guard that the weak subscriptions still deliver events.

Verified by reverting GradientBrush.cs to its current state and re-running: the five leak/Reset tests fail, and the two invalidation guards pass. With the fix all 7 pass, and the full Controls.Core.UnitTests suite is green (5654 passed, 0 failed, 30 skipped).

Issues Fixed

The leak scanner filed the shared-collection retention four separate times; all four describe the same two code paths fixed here.

Fixes #37581
Fixes #38164
Fixes #38179
Fixes #38283

[github-actions]

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[dotnet-policy-service]

Hey there @@wbv-profile! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[dotnet-policy-service]

Hey there @wbv-profile! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[wbv-profile]

@dotnet-policy-service agree

[kubaflo]

Closing this alternate implementation in favor of the existing inflight/current fixes: #37893 (shared collection retention), #36746 (Clear() cleanup), and #36808 (the compatible reference-identity comparer).

The current patch also has two concrete ownership problems in GradientBrush.cs: DetachStop uses collection.Contains(stop) at line 149, but GradientStop.Equals compares values, so removing one of two distinct equal-color/equal-offset stops leaves the removed stop subscribed and parented. Separately, the finalizer calls DetachAllStops, which unconditionally clears Parent at line 164; collecting an earlier brush that shared the collection can therefore clear the parent assigned by a still-live successor brush.

This addresses real problems still visible on main, but we should reuse the existing inflight fixes for any backport rather than merge another implementation with these regressions. Additional regression cases can be carried forward as test-only improvements. Thanks for the detailed leak coverage.