Fix broken listener code in WindowScope

[mattkae]

What's new?

WindowScope is an InheritedModel that is rebuilt with the same controller instance whenever the controller notifies its listeners. Its updateShouldNotify and updateShouldNotifyDependent compared the live controller's values against oldWidget.controller's values, but since the controller is the same object across rebuilds, those comparisons were always false and dependents never rebuilt.

Capture a snapshot of each aspect value in the WindowScope constructor so the old and new widgets hold independent values that can be compared to detect which aspects changed. The per-aspect value computations are extracted into static helpers shared with the *Of accessors.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request updates WindowScope to capture snapshots of its controller's aspect values at construction time, enabling proper change detection in updateShouldNotify and updateShouldNotifyDependent when the same controller instance notifies of changes. It also introduces helper methods to compute these aspect values and adds a test verifying that dependents rebuild upon controller notifications. The review feedback highlights several violations of the Flutter Repository Style Guide, pointing out that private members and helper methods should use triple-slash (///) comments instead of double-slash (//) comments for public-quality documentation.

[loic-sharma]

Copying my message from the original PR: #189061 (comment)

This isn't a blocker for this PR though, please feel free to land this without addressing this comment.

---

I see that all our window widgets wrap the controller in a ListenableBuilder to ensure WindowScope rebuilds when the controller changes:

flutter/packages/flutter/lib/src/widgets/_window.dart

Lines 1584 to 1590 in 7152f2b

	return ListenableBuilder(
	listenable: controller,
	builder: (BuildContext context, Widget? widget) => WindowScope(
	controller: controller,
	child: View(view: controller.rootView, child: child),
	),
	);

A drawback of this approach is that if an app developer - for some reason - decides to create a WindowScope manually but forgets the ListenableBuilder, that WindowScope will have stale values.

Would it make sense to instead have WindowScope create the ListenableBuilder itself? That would ensure that you cannot use WindowScope incorrectly.

[mattkae]

Yes, although I also dislike that WindowScope has a public API to begin with, since it really has no reason to be created by someone else most of the time :)

[loic-sharma]

LGTM, thanks!