avm2: Prevent mouse picking from targeting a loader's root object

[ZingBallyhoo]

Modifies the logic added in #16334 to also consider the case where the root object of a loader is no longer in the loader's hierarchy. This is common when using Event.COMPLETE + event.target.content instead of adding the loader itself as a child.

[ZingBallyhoo]

The new logic also makes stage.mouseChildren = false work so I added a test for it as a bonus.

[ZingBallyhoo]

cc @adrian17

From discord,

// The caller does not expect the stage to be the target, instead return
// None.

For context, there is a lot of code in handle_input_event that interprets None to mean the stage is the pick/hover target.

ruffle/core/src/player.rs

Line 1394 in 4c68a99

context.stage.as_interactive()

ruffle/core/src/player.rs

Line 1534 in 4c68a99

new_over_object.unwrap_or_else(|| context.stage.into()),

ruffle/core/src/player.rs

Line 1698 in 4c68a99

events.push((context.stage.into(), event));

ruffle/core/src/player.rs

Line 1710 in 4c68a99

events.push((context.stage.into(), event));

ruffle/core/src/player.rs

Line 1743 in 4c68a99

events.push((context.stage.into(), event));

ruffle/core/src/player.rs

Line 1756 in 4c68a99

events.push((context.stage.into(), event));

Maybe it would be fine to treat the stage more like a regular object in handle_input_event and let it flow into context.mouse_data.hovered etc but I am not equipped to determine if that could be correct :). So at least for now run_mouse_pick is set up to fiddle with the result to keep the calling logic the same.

The use of combine_with_parent(context.stage.into()) here now instead of the previous custom logic allows the is_root detection to be shared + make stage.mouseChildren = false work now too.

The only cases the stage->None logic runs is:
mouse is over root object - root objects can never be picked -> shared combine logic gives stage as the target
OR
mouse is over child of the stage - stage.mouseChildren = false -> shared combine logic gives stage as the target
both cases are from this one line of code

[adrian17]

LGTM, just sent some minor code-style notes.

[adrian17]

Thank you!