size_changed signal doesn't work on root viewport when using custom scenetree

[blipk]

Godot version

3.4.4.stable.arch_linux

System information

Arch

Issue description

Connecting to the size_changed signal on get_tree().get_root() does not work if using a custom SceneTree loaded via the -s command line argument

Steps to reproduce

• Set a custom SceneTree via command line args (Project Settings -> Editor -> Main Run Args) e.g. -s SceneTreeCustom.gd
• Try to connect to size_changed signal on viewport

extends SceneTree
class_name SceneTreeCustom
func _initialize():
	_on_viewport_resized()
	root.connect("size_changed", self, "_on_viewport_resized")
func _on_viewport_resized():
	prints("Resizing:", root.size)

or via another script

func _ready():
	_on_viewport_resized()
	get_tree().root.connect("size_changed", self, "_on_viewport_resized")
func _on_viewport_resized():
	prints("Resizing:", get_tree().root.size)

• Nothing is printed when resizing the window
• It will work if connecting to the signal without the custom scene tree cmdline arg
• I also tried get_tree().get_root() and get_viewport() and it did not work

Minimal reproduction project

No response

[blipk]

Screen size is not always the same as viewport but this works for what I needed to do:

extends SceneTree
class_name SceneTreeCustom
func _initialize():
	self.connect("screen_resized", self, "_on_screen_resized")
func _on_screen_resized():
	prints("Resizing window:", get_root().size, OS.get_window_size())