Defensive fixes for a UIViewPropertyAnimator crash

[lukas-cap]

Platforms Impacted

• iOS
• macOS

Description of changes

Binary change:

Before	After
31,642,171 bytes	31,639,099 bytes

Problem

Clients can crash if they make certain bottom sheet modifications during UIViewPropertyAnimator completion callbacks.

Cause

UIViewPropertyAnimator has a bug / undocumented behavior - when it's calling out to its completion handlers, the animator reports isRunning==true and it's in a private UIViewAnimatingState (value 5). When it's in this state, calling finishAnimation will cause it to throw an exception.

We have a completeAnimationsIfNeeded helper function that frequently gets called on bottom sheet environment changes, like the host view size / safe area. This helper checks the isRunning prop to make sure it's only trying to force complete animators that are running. But if an animator is in the process of completing, and the client reacts to that completion in a way that triggers completeAnimationsIfNeeded synchronously, we can end up triggering the exception.

Fix

Two fixes to be safe, but either would work to mitigate the problem:

1. Remove our reference to the current animator as soon as possible. This eliminates most / all possibilities for this bug, because we'll bail early in our completeAnimationsIfNeeded helper as it no longer has an animator to act on.
2. Add an additional check to completeAnimationsIfNeeded - in addition to checking isRunning, let's also make sure state == .active. Documentation says this is always true, but it's clearly not.

Verification

Force crash condition by modifying safe area in animator completion. Verify crash doesn't occur anymore with the fixes.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow

[anandrajeswaran]

"Documentation says this is always true" - good place for a link in the PR notes

[harrieshin]

do you mind cherry picking this to main_0.9 branch too please?

[lukas-cap]

"Documentation says this is always true" - good place for a link in the PR notes

Only in the code docs unfortunately:
// Running indicates that the animation is running either in the forward or the reversed direction.
// The state of a running animation is always active.
var isRunning: Bool { get }