Unable to access the HTML & CSS in the sidebar

I followed the instructions in the documentation here: https://extensionworkshop.com/documentation/develop/debugging/#debugging-sidebars

but I only see a generated background page and no sidebar. There is no page select icon available.

Yeah, that works only when you load addon page into the sidebar. Loading a 3rd party page there won’t show up.
I can’t think of a way to debug it, nothing I try works.
All I can think of is a workaround, load the page into normal tab and use mobile view with custom dimensions to emulate sidebar-like view:

Hi Juraj,

You’re thee most helpful user in this forum!

Thank you so much for your help.

Kind regards,