[ Widget Preview ] Improve widget inspector support for widget previews

[bkonyi]

In order to prevent the widget inspector from being able to select and inspect elements of the widget previewer's scaffolding, this commit consists of two notable changes:

1. A debugWillManuallyInjectWidgetInspector property has been added to WidgetsBinding. Setting this property to true before running the application will prevent WidgetsApp from injecting a WidgetInspector instance into the widget tree, even if the widget inspector is enabled. This means that the widget inspector will not be able to select and highlight widgets by default, requiring WidgetInspector to be manually wrapped around widget trees that should be inspectable.

2. The widget_preview_scaffold template has been updated to set debugWillManuallyInjectWidgetInspector to true by default, and to wrap individual previews provided by the developer with an instance of WidgetInspector to restrict widget inspection to the contents of the preview.

This change also includes a minor bug fix for situations where WidgetInspector is inserted into an unconstrained context. Previously, the WidgetInspector's _InspectorOverlay would attempt to take up as much space as possible, causing an overflow. To fix this, the _InspectorOverlay is wrapped with Positioned.fill(...) to force it to take on the same size as its parent Stack.

Work towards #166423

Demo:

Screen.Recording.2025-04-29.at.3.50.56.PM.mov

[bkonyi]

FYI @Piinks, there's a minor change to the framework needed to support this behavior.

[bkonyi]

FYI @elliette

[jyameo]

lgtm!

[elliette]

Why can't we use the existing debugShowWidgetInspectorOverride for widget previews instead?

[bkonyi]

We do, but we also need to make sure that debugShowWidgetInspectorOverride == true doesn't result in an instance of WidgetInspector being inserted by WidgetsApp.

This test is simply testing that debugWillManuallyInjectWidgetInspector is able to disable the creation of WidgetInspector by WidgetsApp.

[elliette]

I'm trying to understand if we can set debugShowWidgetInspectorOverride to false to prevent the widget inspector from being injected into the WidgetsApp (how is the behavior of debugShowWidgetInspectorOverride different than debugWillManuallyInjectWidgetInspector)?

[bkonyi]

We can, but the problem is we also want to allow for the manually created WidgetInspector widgets to be removed from the widget tree when widget inspection isn't enabled. That's done by listening to the value of debugShowWidgetInspectorOverride, which DevTools updates through a service extension to enable / disable the inspector. debugExcludeRootWidgetInspector is used to differentiate between the root widget inspector simply being disabled due to widget inspection not being currently enabled vs us never wanting the root widget inspector to be inserted into the widget tree.

[Piinks]

LGTM with the suggested name change!

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autosubmit label was removed for flutter/flutter/168013, because - The status or check suite Mac tool_integration_tests_1_5 has failed. Please fix the issues identified (or deflake) before re-applying this label.