Pausing tree disables white-listed Area2D's signals

[markdibarry]

Godot version

3.5 RC4

System information

Windows 10

Issue description

When pausing the scene tree, and an Area2D is white-listed to process while paused, the body_entered signals are not fired.

2022-06-18.10-55-28.mp4

Steps to reproduce

1. Create a scene with a white-listed branch.
2. Have the branch contain a moveable body and Area2D
3. Set Area2D's body_entered signal to notify when body has entered.
4. pause tree and move body over Area2D

Minimal reproduction project

CollisionBug.zip

[Zireael07]

I believe this has already been reported.

[markdibarry]

I looked through open issues and #45334 but I couldn't find it.

[Zireael07]

#61420 I think is what I remember. Not all issues are listed on the tracker

[markdibarry]

Ah! That one is specific to 4.0, and has a different result with the new pause system, causing the signal to be removed entirely or throw errors, depending on the node tree order.

This one is for 3.x, where pausing the scene tree stops Area2Ds from working regardless of whether or not they're white-listed.

[lapspider45]

Workaround: call Physics2DServer.set_active(true) after pausing the tree.

A side effect of get_tree().paused = true is to disable the physics engine. It is documented, just not very clearly:
https://docs.godotengine.org/en/stable/classes/class_scenetree.html#class-scenetree-property-paused

[markdibarry]

Hm. This does resume the physics server, but still seems like an unintentional side-effect. It's a far-reaching workaround, because with the pause system currently, it means either:

1. White-listed nodes don't behave as white-listed (physics disabled for all).
2. Paused nodes don't behave as paused (physics enabled for all).

Unless I'm misunderstanding and the workaround only resumes the Physics server for nodes set to process while paused.

[Calinou]

I don't think this can be resolved unless something like #46191 is backported in a backwards-compatible manner, which sounds too difficult to do.

[markdibarry]

@Calinou The mentioned rewrite introduces it's own set of (worse?) Area2D bugs (mentioned a few comments above), but you're absolutely right. It's probably not worth spending the effort to backport the fix to 3.x in the long-run and just focus on 4.0. It seems like, for now, the best solution is either a custom pause system in 3.x or wait for Area2D to be fixed in 4.0.

As reference for anyone else who needs to be able to pause in 3.x without this bug, nodes have the following methods you can use to emulate a custom system, by iterating through a branch:
set_physics_process()
set_physics_process_internal()
set_process()
set_process_internal()
set_process_input()
Keep in mind it can be brittle so node reparenting should be kept to a minimum.

[markdibarry]

Closing since it can't be fixed in 3.x