[luanpotter]

Description

Make transparent hit-test opt-in via DeferHitTestToComponents.

FlameGame used to answer containsEventHandlerAt by walking the component tree and reporting a hit only where a component implemented PointerInputCallbacks, which was an expensive recursive check for a feature I'd imagine nearly no one uses.

This changes the default - it now reports a hit everywhere, like the base Game class, and the tree walk is opt-in via a mixin. This only affects games built with HitTestBehavior.deferToChild, since opaque and translucent never consult the game. If you use deferToChild to let events reach widgets behind the GameWidget, add the mixin, or override containsEventHandlerAt with a cheaper rule of your own.

I think this is a saner alternative to @spydon 's #3982 , as we discussed before. The ultimate control is in the user hands to implement this in a more effective way w/o the mixin. This also uses the new marker interface to avoid having to list all types of click handlers.

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.

Migration instructions

Only applicable if you do want this specific behaviour; add the mixin to your FlameGame:

// Before: implicit, and paid for on every hit test under deferToChild
class MyGame extends FlameGame {}

// After
class MyGame extends FlameGame with DeferHitTestToComponents {}

[spydon]

Lgtm, this is better indeed!