Add breaking change doc for Visibility widget changes (#12488)

_Description of what this PR is changing or adding, and why:_
This PR adds a breaking change document around `Visibility` changes that
landed in stable release 3.35.

_Issues fixed by this PR (if any):_
Fixes [#174652](flutter/flutter#174652)

_PRs or commits this PR depends on (if any):_
Does not depend on any PRs that have not already landed in stable.

Related PRs:
flutter/flutter#159133

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Author: Renzo-Olivares

src/content/release/breaking-changes/index.md

@@ -71,6 +71,7 @@ They're sorted by release and listed in alphabetical order:
 * [The `Form` widget no longer supports being a sliver][]
 * [Flutter now sets default `abiFilters` in Android builds][]
 * [Merged threads on macOS and Windows][]
+* [The `Visibility` widget is no longer focusable by default when `maintainState` is enabled][]
 
 [Component theme normalization updates]: /release/breaking-changes/component-theme-normalization-updates
 [Deprecate `DropdownButtonFormField` `value` parameter in favor of `initialValue`]: /release/breaking-changes/deprecate-dropdownbuttonformfield-value
@@ -80,6 +81,7 @@ They're sorted by release and listed in alphabetical order:
 [The `Form` widget no longer supports being a sliver]: /release/breaking-changes/form-semantics
 [Flutter now sets default `abiFilters` in Android builds]: /release/breaking-changes/default-abi-filters-android
 [Merged threads on macOS and Windows]: /release/breaking-changes/macos-windows-merged-threads
+[The `Visibility` widget is no longer focusable by default when `maintainState` is enabled]: /release/breaking-changes/visibility-maintainfocusability
 
 <a id="released-in-flutter-332" aria-hidden="true"></a>
 ### Released in Flutter 3.32

src/content/release/breaking-changes/visibility-maintainfocusability.md

@@ -0,0 +1,65 @@
+---
+title: The Visibility widget is no longer focusable by default when maintainState is enabled
+description: >-
+  The Visibility widget by default no longer implicitly retains focusability
+  for its child when maintainState is enabled.
+---
+
+## Summary
+This change was introduced to fix an issue
+where an `IndexedStack`s hidden children would be focusable with keyboard events
+(see [issue](https://github.com/flutter/flutter/issues/114213))
+due to the underlying `Visibility` widgets default behavior.
+
+## Description of change
+The core change is the `Visibility` widget is no longer focusable by default
+when `maintainState` is enabled.
+A new flag, `maintainFocusability`, must be set to true with `maintainState`
+for a hidden widget to remain focusable.
+
+## Migration guide
+If your app has a `Visibility` widget that does not set `maintainState` to true, 
+then no changes are required.
+
+If your app has a `Visibility` widget that sets `maintainState` to true
+and you relied on the previous default behavior 
+that allowed you to focus your hidden widget,
+you will need to set `maintainFocusability` to true.
+
+Code before migration:
+
+```dart
+child: Visibility(
+    maintainState: true,
+    child: SomeWidget(),
+)
+```
+
+Code after migration:
+
+```dart
+child: Visibility(
+    maintainState: true,
+    maintainFocusability: true,
+    child: SomeWidget(),
+)
+```
+
+## Timeline
+
+Landed in version: 3.34.0-pre<br>
+In stable release: 3.35
+
+## References
+
+API documentation:
+
+* [`Visibility`]({{site.api}}/flutter/widgets/Visibility-class.html)
+
+Relevant issues:
+
+* [Issue 114213]({{site.repo.flutter}}/issues/114213)
+
+Relevant PRs:
+
+* [PR 159133: Add flag to exclude focus for hidden children in Visibility, maintainFocusability. Set maintainFocusability to false in IndexedStack]({{site.repo.flutter}}/pull/159133)