fix: Recognize every input callbacks mixin when hit testing

[luanpotter]

Description

Follow up from #3986 (comment)

FlameGame.containsEventHandlerAt backs GameRenderBox.hitTestSelf, which decides (for now) whether the game takes part in the hit test when the GameWidget uses a HitTestBehavior other than opaque. It enumerated some callbacks mixins and was missing five others: LongPressCallbacks, TertiaryTapCallbacks, ScrollCallbacks, PointerMoveCallbacks and HoverCallbacks.

Before, components using any of those would silently received no events under deferToChild or translucent.

Rather than extending the list and the problem, this adds two marker interfaces:

• InputCallbacks: anchors the whole family, much like Event does for the event classes.
• PointerInputCallbacks implements InputCallbacks: the positional/pointer subset, i.e. the mixins whose events carry a position and can therefore take part in hit testing.

This serves this and other future purposes, and is public API for users to access and write generic code on top.

I will still explore (after this) following the direction I proposed in #3982: FlameGame stops overriding containsEventHandlerAt altogether - the game reports a hit anywhere within its bounds, and per-component transparency becomes an explicit opt-in user override written against PointerInputCallbacks using this interface:

@override
bool containsEventHandlerAt(Vector2 position) =>
    componentsAtPoint(position).any((c) => c is PointerInputCallbacks);

Regardless, this is a great shape to have the callbacks in, so I am extracting this first.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[luanpotter]

this is the fix

[Copilot]

Pull request overview

This PR fixes FlameGame.containsEventHandlerAt / GameWidget hit-testing under HitTestBehavior.deferToChild and translucent by introducing marker interfaces for input-callback mixins and using them for pointer-position-aware hit testing.

Changes:

• Add InputCallbacks and PointerInputCallbacks marker interfaces and export them as public API.
• Update pointer/positional callback mixins to implement PointerInputCallbacks, and simplify FlameGame.containsEventHandlerAt to check for that marker.
• Add regression tests and update GameWidget documentation to reflect the new behavior.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/flame/test/game/contains_event_handler_at_test.dart	Adds regression coverage for containsEventHandlerAt and GameWidget hit-testing (including LongPressCallbacks under deferToChild).
packages/flame/lib/src/game/flame_game.dart	Replaces a hard-coded list of callback mixins with a PointerInputCallbacks marker check for hit testing.
packages/flame/lib/src/events/callbacks/input_callbacks.dart	Introduces the root marker interface for all input callback mixins.
packages/flame/lib/src/events/callbacks/pointer_input_callbacks.dart	Introduces the positional/pointer subset marker used for hit testing.
packages/flame/lib/src/events/callbacks/tap_callbacks.dart	Implements PointerInputCallbacks on TapCallbacks.
packages/flame/lib/src/events/callbacks/secondary_tap_callbacks.dart	Implements PointerInputCallbacks on SecondaryTapCallbacks.
packages/flame/lib/src/events/callbacks/tertiary_tap_callbacks.dart	Implements PointerInputCallbacks on TertiaryTapCallbacks.
packages/flame/lib/src/events/callbacks/drag_callbacks.dart	Implements PointerInputCallbacks on DragCallbacks.
packages/flame/lib/src/events/callbacks/double_tap_callbacks.dart	Implements PointerInputCallbacks on DoubleTapCallbacks.
packages/flame/lib/src/events/callbacks/scale_callbacks.dart	Implements PointerInputCallbacks on ScaleCallbacks.
packages/flame/lib/src/events/callbacks/scroll_callbacks.dart	Implements PointerInputCallbacks on ScrollCallbacks.
packages/flame/lib/src/events/callbacks/pointer_move_callbacks.dart	Implements PointerInputCallbacks on PointerMoveCallbacks (covering HoverCallbacks transitively).
packages/flame/lib/src/events/callbacks/long_press_callbacks.dart	Implements PointerInputCallbacks on LongPressCallbacks.
packages/flame/lib/events.dart	Exports the new marker interfaces as part of the public events.dart surface.
doc/flame/game_widget.md	Documents that pointer hit-testing treats PointerInputCallbacks components as interactive.

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[erickzanardo]

Left a question, but LGTM either way

[erickzanardo]

Shouldn't this and the other interface define methods?

[luanpotter]

they don't actually share any methods or fields atm - this is just a marker interface for type-checking purposes