Add v3.126 release notes

Author: DanTup

_releases/v3.126.md

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+---
+title: v3.126
+sort_order: v0003_0126
+excerpt: "Widget Preview improvements, Flutter sidebar after sleep fix, Test runner improvements, ..."
+---
+
+# v3.126.0 _2025-12-29_
+
+## Flutter
+
+- [#5840](https://github.com/Dart-Code/Dart-Code/issues/5840)/[#5814](https://github.com/Dart-Code/Dart-Code/issues/5814): To improve startup performance, the widget preview now starts on first access rather than at startup. This can be controlled with a new setting `dart.flutterWidgetPreview` that accepts `"startLazily"` (default), `"startEagerly"` (starts at startup when a Flutter project is present) and `"disabled"`.
+
+## Testing
+
+- [#5842](https://github.com/Dart-Code/Dart-Code/issues/5842)/[#5833](https://github.com/Dart-Code/Dart-Code/issues/5833): When running a batch of tests from the Test Explorer requires multiple sequential debug sessions, a progress notification will be shown showing the number of sessions and the ability to cancel the entire batch.
+- [#5831](https://github.com/Dart-Code/Dart-Code/issues/5831)/[#5834](https://github.com/Dart-Code/Dart-Code/issues/5834): Deleting a test no longer causes the next test marker to move to the incorrect line.
+
+## Misc
+
+- [#5841](https://github.com/Dart-Code/Dart-Code/issues/5841): A warning is now shown that Dart 3.1 / Flutter 3.13 will become unsupported in a future release.
+- [#5848](https://github.com/Dart-Code/Dart-Code/issues/5848): Several unexpected conditions that trigger extension restarts will now show warning notifications prompting to file an issue for investigation.
+- [#5830](https://github.com/Dart-Code/Dart-Code/issues/5830): ~~Updated to LSP client v10.0.0-next.18 and enabled `delayOpenNotifications` for improved performance.~~ This change was reverted pending a fix in the VS Code LSP client.
+
+## Features/Fixes Enabled with Future SDKs
+
+The following items depend on changes to the Dart/Flutter SDKs that have not yet been released to stable channels. They may be available to test in the current beta/master channels and will show up in future SDK releases.
+
+- [#5832](https://github.com/Dart-Code/Dart-Code/issues/5832): `formatOnType` no longer triggers when typing `;` inside a string literal.
+- [#5602](https://github.com/Dart-Code/Dart-Code/issues/5602): `formatOnType` no longer triggers when typing `;` inside an enhanced enum without a constructor, which would cause the semicolon to be removed.
+- [#5808](https://github.com/Dart-Code/Dart-Code/issues/5808): The Flutter Sidebar no longer stops responding after the PC wakes from sleep. Instead, a "Reconnecting..." notification will be shown while re-connecting to DTD.
+