Return null from ComposeAccessible.getAccessibleContext when the node has been removed

[m-sasha]

The long story of this PR is this:

I reproduced the printed exception in JetBrains/compose-multiplatform#3728 and noticed that we don't report to the accessibility system anywhere when a node has been removed. So I thought it wasn't surprising that it would continue calling functions on it, which would then throw that exception.

So I set out to report node addition/removal by correctly firing AccessibleContext.ACCESSIBLE_CHILD_PROPERTY property change events. When that didn't do anything, I realized that ComposeAccessibleComponent doesn't have the child/parent hierarchy set up correctly, and thought that perhaps this is confusing the system.

Specifically rootAccessible in AccessibilityControllerImpl (which is only the root for a single SemanticsOwner, so it's not really the root) doesn't report having a parent, even though ComposeSceneAccessibleContext returns it from getAccessibleChild. So I fixed that. But unfortunately that didn't help either.

Then I decided to see what exactly the system does when it receives an ACCESSIBLE_CHILD_PROPERTY property change event. Turns out that AXChangeNotifier (the accessible listener on macOS) doesn't even listen to that event, and its (native) ptr is null altogether for some reason.

So then I just gave up and implemented this PR. It simply marks ComposeAccessibles as removed when they are removed, and their getAccessibleContext then returns null. In CAccessibility.java, all the getter functions check the return value of getAccessibleContext and return null if it returns null.

If we want to properly report ACCESSIBLE_CHILD_PROPERTY events and have the parents set up correctly, I can submit a separate PR, or just keep it stashed for now.

Proposed Changes

When the semantics node of a ComposeAccessible is removed, mark it as removed, and have it return null from getAccessibleContext.

Fixes: JetBrains/compose-multiplatform#3728

[Walingar]

Am I right that tree will be recreated after a while and ComposeAccessible will be collected? Otherwise it looks like a leak

[m-sasha]

Not sure I understand what you mean. The node is marked as removed=false when it's no longer in the tree. I'm not storing it anywhere it wasn't before.

sun.lwawt.macosx.CAccessible will probably keep storing a reference to it, but only until some other node becomes "active" (it only stores one Accessible).

[Walingar]

Yes, I am talking about sun.lwawt.macosx.CAccessible, so I hope that ComposeAccessible won't be stored there forever

[Walingar]

But anyway I also investigated this behavior when working on the issue with non-working accessibility in popups. I thought that the issue is that we are not notifying system about tree changes. But system doesn't even listen for the events.
So, for me this solution works for now, but I am afraid about possible leak