Add Flutter Linux merged threads migration announcement (#12534)

Raise awareness that Flutter 3.39/3.41 merged the
UI/platform threads on Linux. This is a significant internal change that
_might_ break apps. If so, we'd like customers to reach out to us on
GitHub so that we can fix bugs, if any.

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: robert-ancell

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

@@ -36,16 +36,18 @@ They're sorted by release and listed in alphabetical order:
 
 ### Not yet released to stable
 
-* [`FontWeight` also controls the weight attribute of variable fonts][]
+* [Merged threads on Linux][]
+* [Stop generating `AssetManifest.json`][]
 * [UISceneDelegate adoption][]
 * [`$FLUTTER_ROOT/version` replaced by `$FLUTTER_ROOT/bin/cache/flutter.version.json`][]
-* [Stop generating `AssetManifest.json`][]
+* [`FontWeight` also controls the weight attribute of variable fonts][]
 * [Deprecate `TextField.canRequestFocus`][]
 
-[`FontWeight` also controls the weight attribute of variable fonts]: /release/breaking-changes/font-weight-variation
-[UISceneDelegate adoption]: /release/breaking-changes/uiscenedelegate
+[Merged threads on Linux]: /release/breaking-changes/linux-merged-threads
 [Stop generating `AssetManifest.json`]: /release/breaking-changes/asset-manifest-dot-json
+[UISceneDelegate adoption]: /release/breaking-changes/uiscenedelegate
 [`$FLUTTER_ROOT/version` replaced by `$FLUTTER_ROOT/bin/cache/flutter.version.json`]: /release/breaking-changes/flutter-root-version-file
+[`FontWeight` also controls the weight attribute of variable fonts]: /release/breaking-changes/font-weight-variation
 [Deprecate `TextField.canRequestFocus`]: /release/breaking-changes/can-request-focus
 
 <a id="released-in-flutter-338" aria-hidden="true"></a>

src/content/release/breaking-changes/linux-merged-threads.md

@@ -0,0 +1,49 @@
+---
+title: Merged threads on Linux
+description: >-
+  Learn about the threading changes on Linux in Flutter 3.39.
+---
+
+## Summary
+
+Flutter 3.39 merges the UI and platform threads by default on Linux.
+
+## Context
+
+Originally, Flutter had separate threads to produce UI frames and
+to interact with the native platform.
+
+The split-thread design prevented Flutter apps and plugins from using Dart FFI
+to interoperate with native APIs that must be called on the platform thread.
+
+## Description of change
+
+Flutter version 3.39 merges the UI and platform threads by default on Linux.
+
+This mirrors all the other platforms, whose threads were merged by default in
+Flutter 3.29 (iOS and Android) and 3.35 (macOS and Windows).
+
+## Migration guide
+
+Merged threads shouldn't affect your app.
+
+If you suspect merged threads have caused regressions to your app,
+please reach out on [Issue 150525][].
+
+## Timeline
+
+Landed in version: 3.39.0-0.1.pre<br>
+In stable release: Not yet
+
+## References
+
+Relevant issue:
+
+* [Issue 150525][]
+
+Relevant PRs:
+
+* [PR 176759][]
+
+[Issue 150525]: {{site.repo.flutter}}/issues/150525
+[PR 176759]: {{site.repo.flutter}}/pull/176759