export_png not exporting figures correctly

This is a continuation of #7621

export_png (and export_svgs for that matter) do not work as expected when attempting to export figures after they were added to a layout curdoc().

Here is the example code:

# -*- coding: utf-8 -*-
'''
After the layout has been added to the root, exporting the png of a fig is
not possible anymore
With "bokeh serve export_test.py", the console output for me is the following:
doc_layout works
fig works
Layout is added to root
doc_layout works
fig throws error
'''
from bokeh.io import curdoc
from bokeh.models import ColumnDataSource
from bokeh.plotting import figure
from bokeh.layouts import layout, widgetbox
from bokeh.io import export_png
from bokeh.models.widgets import Button


def download():
    try:
        export_png(doc_layout, filename="export doc_layout.png")
        print('doc_layout works')
    except Exception as ex:
        print('doc_layout throws error:', ex)
    try:
        export_png(fig, filename="export fig.png")
        print('fig works')
    except Exception as ex:
        print('fig throws error:', ex)


source = ColumnDataSource({'x': [2, 3, 4], 'y': [0.5, 1.5, 2.5]})
fig = figure(plot_width=1000, plot_height=300)
fig.hbar(y='y', right='x', height=0.4, source=source)

button = Button(label="Download")
button.on_click(download)
doc_layout = layout([[widgetbox(button)], [fig]], sizing_mode='fixed')

#download()  # Test 1

print('Layout is added to root')
curdoc().add_root(doc_layout)
curdoc().title = "Export PNG test"

#download()  # Test 2

I start the server with bokeh serve export_test.py. After I click the Download button, the following output is generated:

You are generating standalone HTML/JS output, but trying to use real Python
callbacks (i.e. with on_change or on_event). This combination cannot work.

Only JavaScript callbacks may be used with standalone output. For more
information on JavaScript callbacks with Bokeh, see:

    http://bokeh.pydata.org/en/latest/docs/user_guide/interaction/callbacks.html

Alternatively, to use real Python callbacks, a Bokeh server application may
be used. For more information on building and running Bokeh applications, see:

    http://bokeh.pydata.org/en/latest/docs/user_guide/server.html

doc_layout works
fig throws error: Message: {"errorMessage":"undefined is not an object (evaluating 'document.getElementsByClassName('bk-root')[0].children[0].getBoundingClientRect')","request":{"headers":{"Accept":"application/json","Accept-Encoding":"identity","Connection":"close","Content-Length":"171","Content-Type":"application/json;charset=UTF-8","Host":"127.0.0.1:62171","User-Agent":"selenium/3.12.0 (python linux)"},"httpVersion":"1.1","method":"POST","post":"{\"script\": \"\\nreturn document.getElementsByClassName('bk-root')[0].children[0].getBoundingClientRect()\\n\", \"args\": [], \"sessionId\": \"5f2d8590-7620-11e8-94ca-bf888776a2b5\"}","url":"/execute","urlParsed":{"anchor":"","query":"","file":"execute","directory":"/","path":"/execute","relative":"/execute","port":"","host":"","password":"","user":"","userInfo":"","authority":"","protocol":"","source":"/execute","queryKey":{},"chunks":["execute"]},"urlOriginal":"/session/5f2d8590-7620-11e8-94ca-bf888776a2b5/execute"}}
Screenshot: available via screen

If I comment out the line download() # Test 1, the correct png is generated. This is only possible before curdoc().add_root(doc_layout). I still cannot save the png of a figure, once it has been included in a layout and added to root. Am I doing someting wrong? Does it have something to do with the new standalone HTML warning?

Software: Bokeh = 0.13.0 Tornado = 5.0.2 Selenium = 3.12.0 Pillow = 5.1.0

[znstrider]

Thank you AzraelDD, your solution works for me.

[bryevdv]

Closing since, as I said the original code now works for me, and also the annoying warning has been silenced. Please reopen or comment if issues are persisting with latest dev versions.