Dev environments no longer load previous version from CDN

[pranshukharkwal]

ALL software version info (bokeh, python, notebook, OS, browser, any other relevant packages)

Bokeh - Installed using an updated fork of https://github.com/bokeh/bokeh
Python - 3.8.3
OS - Windows 10
Browser - Google Chrome

Description of expected behavior and the observed behavior

When the environment variable "BOKEH_RESOURCES=inline" is not set, the HTML uses remote CDN BokehJS resources, and it uses a wrong URL which is
<https://cdn.bokeh.org/bokeh/release/bokeh-0+unknown.min.js
HTML fails to get these resources, and results in an empty page.

Complete, minimal, self-contained example code that reproduces the issue

If the environment variable is already set, then unset it. Then run the command.

python examples/plotting/file/iris.py

Stack traceback and/or browser JavaScript console output

Failed to load resource: the server responded with a status of 403 ()
iris.html:21 Uncaught ReferenceError: Bokeh is not defined
at iris.html:21
iris.html:49 Uncaught ReferenceError: Bokeh is not defined
at HTMLDocument.fn (iris.html:49)

Screenshots or screencasts of the bug in action

PS : I checked with example plots, but I guess it would have the same problem with other plots as well.

[pranshukharkwal]

cc : @bryevdv

[bryevdv]

Setting this as discussion because I am not sure what should be done here.

Previously, when using CDN resources with a "dev" environment, Bokeh would fall back to whatever the last released version of BokehJS on CDN is. Sometimes this "works" and sometimes it does not. In the Notebook, a big banner warning is displayed waring that "this configuration is not supported and may not work".

Recently, something disturbed that. I had also noticed this but not gotten around to making an issue (thank you @pranshukharkwal). I would like to understand the change, but I am not sure that it should be "fixed" in the sense that I am not sure it's good to return to the previous behavior. I think perhaps it would be better to detect if a "dev" version of Bokeh is being used and warn (in Python or a browser alert, or both) that local dev should use inline or server resources in order to match up.

Thoughts @bokeh/dev ?

[mattpap]

It looks like this is similar or the same issue as we had with mismatched bokeh and bokehjs versions, though I can't find the issue now (maybe there was none created).