Particles aren't emiting when scale_curve begins from zero

[sash-rc]

Godot version

v3.6.beta3.official [21ab700]

System information

Linux Ubuntu 22.04, GeForce 1060

Issue description

Particles stops emitting when the first point of Scale Curve in Process Material is zero, like with "Linear preset".
Another fact is that setting flag_disable_z or flag_disable_z cancels this effect (emitting works).

The same setup in Godot_v3.5.2-stable works without problems.

Steps to reproduce

Just any Particles set up as described in Issue description

Minimal reproduction project

N/A

[Calinou]

This occurred in 4.x at some point and was resolved by #78021.

cc @lawnjelly – this may be related to interpolation changes.

[lawnjelly]

Needs MRP. Please always include MRP, no matter how simple to you, it may take time to reproduce / not reproduce on another machine etc.

EDIT:
I've now created an MRP but please in future always try to make one as it makes our lives much easier, and makes the issue more likely to be looked at:
sash_partlcles.zip

I don't think I did any changes directly to Particles for 3.6 (or 3.5 actually, there were no changes to GPUParticles if I remember right, only CPUParticles). 3.6 has changes to CPUParticles2D only.

There were some general changes for interpolation but I don't think this would be likely to cause this.

UPDATE:
This regression has existed since 3.6 beta 1 at least. Will try and bisect.

[lawnjelly]

Bisected to #53852 . @Calinou .
Buck passed! 😁

[sash-rc]

Needs MRP. Please always include MRP

Sorry, I just thought reproduction steps are very basic and don't require dedicated project.
But ok, will do my best next time.

[akien-mga]

We should solve this before the next beta. What approach do we want to take? Revert #53852 or is there a way to keep both cases working?

[akien-mga]

Fixed by #81902.

[lawnjelly]

Strangely I couldn't seem to get this bug to reproduce in master (despite having the same code here), so I've shelved master PR for now.