As far as I know, you can't do that automatically since the bars would end up being part of your project. You'd have to turn off all of the automatic scaling and use the Viewport.size_changed() signal to resize your game and sidebars individually. This code does something similar with three textures, representing two horizontal sidebars and a game.
extends Node2D
var ideal_size = Vector2.ONE * 400
func _ready() -> void:
var trs = []
var tr = TextureRect.new()
tr.texture = preload('res://icon.png')
tr.expand = true
tr.rect_min_size = ideal_size
trs.append(tr)
trs.append(tr.duplicate())
trs.append(tr.duplicate())
var p = Vector2.ZERO
for t in trs:
t.rect_position = p
add_child(t)
p.x += ideal_size.x
rescale()
var vp := get_viewport()
vp.connect('size_changed', self, 'rescale')
func rescale():
var vp := get_viewport()
var sc = vp.size.y / ideal_size.y
scale = Vector2.ONE * sc
var ts = ideal_size.x * sc * 3
position.x = (vp.size.x - ts) / 2.0