Appending layout regression

[bryevdv]

@mattpap not sure if this is covered by other issues but I am making a new one specifically because it is a regression. This code works in 0.12.5 and 0.12.4 (plots are added below) but does not work in master (plots stack on top of each other)

from tornado.ioloop import IOLoop
import numpy as np

from bokeh.application.handlers import FunctionHandler
from bokeh.application import Application
from bokeh.plotting import figure
from bokeh.server.server import Server
from bokeh.models.layouts import VBox

io_loop = IOLoop.current()
layout = VBox()


def modify_doc(doc):
    doc.add_root(layout)
    doc.add_periodic_callback(add_plot, 3000)


def add_plot():
    print("add plot")
    x = np.linspace(0, 4 * np.pi, 1000)
    y = np.sin(x)
    plot2 = figure(y_range=(-1.2, 1.2), plot_width=300, plot_height=300)
    plot2.line(x, y, line_width=2, legend="x")
    layout.children.append(plot2)

bokeh_app = Application(FunctionHandler(modify_doc))
server = Server({'/': bokeh_app}, io_loop=io_loop)
server.start()


if __name__ == '__main__':
    print('Opening Bokeh application on http://localhost:5006/')
    io_loop.add_callback(server.show, "/")
    io_loop.start()

[bryevdv]

Just confirming that this is not fixed by any work up to 0.12.6dev7