godot: Using class_name in a newly created script does not register it in the node creation dialog, if under addons/ (unless part of an enabled editor plugin)

Godot 3.1.1 Windows 10 64 bits

I suspect there is a scenario where it doesnt get registered at all. I was writing a new prototype this evening and the script still hasn’t registered.

Here is what I remember doing:

1. Create a new project (no new scene)
2. Create a subfolder addons/zylann.scatter/
3. Right-click in the FileSystem dock and choose “New Script” into that folder
4. Write the following (I wrote it by hand so might have had a few errors while doing so, but the end result was the same):

tool
class_name Scatter3D
extends Spatial

var _scenes = []

func _ready():
	# TODO Temporary test
	_scenes.append(load("res://props/placeholder_tree.tscn"))

1. Save the script (still no scene in the project, I don’t use the default Ctrl+S, instead I save the script only)
2. Create a new scene, open the node creation dialog, search for Scatter: it’s nowhere to be found, despite the class name being recognized by the script editor and saved in project.godot.

Even after restarting the editor, the node can’t be found. No particular messages in the console. Did I miss something obvious?

Here is the project, apparently you should be able to see directly what happens, since restart doesnt fix it: Scatter.zip

Edit: It would seem any script I create under addons/zylann.scatter/ never get registered, but they do if I they are under the project root Oo According to TheDuriel it’s intented. Registration works in other systems, just not in the node dialog. I guess it’s because plugins can be enabled/disabled, so it would make sense for them to no longer clutter the dialogs. However, it’s not documented…