Replace hardcoded platform identity checks in the framework with capability-based queries

[jmagman]

Description

Currently, the Flutter framework relies on hardcoded platform identity checks (e.g., defaultTargetPlatform or Theme.of(context).platform) to determine behavior. This creates friction for out-of-tree custom embedders, who are forced to spoof their OS identity to inherit basic capabilities.

Proposal:

• Replace hardcoded platform identity checks within the core framework with capability-based queries (e.g., supportsPhysicsBasedScrolling).
• Establish and document a mechanism for custom embedders to declare their supported capabilities to the framework.
• Allow existing Flutter-maintained platforms (like Android) to serve as an extensible "base." This would allow custom platforms that share most (but not all) behaviors with a supported platform to inherit its capabilities and deviate slightly only where needed.

Here are several examples:

1. FlexibleSpaceBar uses Theme.of(context).platform to decide whether to center titles.
   

   flutter/packages/flutter/lib/src/material/flexible_space_bar.dart

   Lines 191 to 200 in 00b14ee

   	bool _getEffectiveCenterTitle(ThemeData theme) {
   	return widget.centerTitle ??
   	switch (theme.platform) {
   	TargetPlatform.android ||
   	TargetPlatform.fuchsia ||
   	TargetPlatform.linux ||
   	TargetPlatform.windows => false,
   	TargetPlatform.iOS || TargetPlatform.macOS => true,
   	};
   	}

2. Drawer relies on platform checks to determine layout, semantics, and system back button presence, ex:
   

   flutter/packages/flutter/lib/src/material/drawer.dart

   Lines 253 to 259 in 00b14ee

   	final String? label = switch (defaultTargetPlatform) {
   	TargetPlatform.iOS || TargetPlatform.macOS => semanticLabel,
   	TargetPlatform.android ||
   	TargetPlatform.fuchsia ||
   	TargetPlatform.linux ||
   	TargetPlatform.windows => semanticLabel ?? MaterialLocalizations.of(context).drawerLabel,
   	};

3. TextMagnifier hardcodes the magnifier implementation by switching on defaultTargetPlatform, returning null on unrecognized platforms.
   

   flutter/packages/flutter/lib/src/material/magnifier.dart

   Lines 47 to 57 in 00b14ee

   	switch (defaultTargetPlatform) {
   	case TargetPlatform.iOS:
   	return CupertinoTextMagnifier(controller: controller, magnifierInfo: magnifierInfo);
   	case TargetPlatform.android:
   	return TextMagnifier(magnifierInfo: magnifierInfo);
   	case TargetPlatform.fuchsia:
   	case TargetPlatform.linux:
   	case TargetPlatform.macOS:
   	case TargetPlatform.windows:
   	return null;
   	}

4. The Radio.adaptive constructor uses a switch statement to return a CupertinoRadio for Apple platforms and defaults to the standard Material radio for all others.
   

   flutter/packages/flutter/lib/src/material/radio.dart

   Lines 518 to 540 in 00b14ee

   	switch (theme.platform) {
   	case TargetPlatform.android:
   	case TargetPlatform.fuchsia:
   	case TargetPlatform.linux:
   	case TargetPlatform.windows:
   	break;
   	case TargetPlatform.iOS:
   	case TargetPlatform.macOS:
   	return CupertinoRadio<T>(
   	value: widget.value,
   	groupValue: widget.groupValue,
   	onChanged: widget.onChanged,
   	mouseCursor: widget.mouseCursor,
   	toggleable: widget.toggleable,
   	activeColor: widget.activeColor,
   	focusColor: widget.focusColor,
   	focusNode: _focusNode,
   	autofocus: widget.autofocus,
   	useCheckmarkStyle: widget.useCupertinoCheckmarkStyle,
   	groupRegistry: _effectiveRegistry,
   	enabled: _enabled,
   	);
   	}

5. Autocomplete assigns keyboard shortcuts by switching on defaultTargetPlatform.
   

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

   Lines 412 to 419 in 00b14ee

   	...switch (defaultTargetPlatform) {
   	TargetPlatform.iOS => _appleShortcuts,
   	TargetPlatform.macOS => _appleShortcuts,
   	TargetPlatform.android => _nonAppleShortcuts,
   	TargetPlatform.linux => _nonAppleShortcuts,
   	TargetPlatform.windows => _nonAppleShortcuts,
   	TargetPlatform.fuchsia => _nonAppleShortcuts,
   	},

[huycozy]

This makes perfect sense! I would like to take some of these, as I did several slightly related issues previously. However, 4/5 widgets here are material, perhaps we should wait until decoupling is done. Meanwhile, I can take a look at Autocomplete first, as it's a framework widget.

[justinmc]

Great idea 👍