bokeh: Plot becomes unresponsive when button widget is appended to layout

Hi,

I have a problem when appending Button widgets to a VBox in a nested layout.

Python: 2.7.12 64-bit Bokeh Version: 0.12.4 Browser: Chrome & Edge OS: Windows 10

The below code reproduces the problem. When the add button is pressed the new Button widget is displayed, however the plot figure stops updating or responding. A small graphics glitch also occurs on the title of the plot. No console or browser errors are reported.

import numpy as np

from bokeh.io import curdoc
from bokeh.layouts import row
from bokeh.models import ColumnDataSource
from bokeh.plotting import figure

# Set up data
x = np.linspace(0, 4*np.pi, 200)
y = np.sin(x)
source = ColumnDataSource(data=dict(x=x, y=y))

# Set up plot
plot = figure()

plot.line('x', 'y', source=source)

# Problem Code:
from bokeh.models.widgets import Button
from bokeh.models.layouts import VBox

button = Button(label="Add")
vbox = VBox(children=[button])

def create_button():
    button = Button(label=str(len(vbox.children)))
    vbox.children.append(button)

button.on_click(create_button)

curdoc().add_root(row(vbox, plot))
curdoc().title = "Sliders"

Thanks in advance for any help on this.

Regards, Simon