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Short answer: This isn’t possible. I requested a feature like this during the prerelease process, and it has been considered to be built in, but, unfortunately Muse is EOL, so we will have to live without this. The discussion about this subject can be found here, if you signed up to the prerelease program: https://forums.adobeprerelease.com/muse/discussion/4776/sidebar-the-forgotten-feature
Why is it not possible, to achieve your design idea?
In Muse, the width of responsive elements is calculated in relation to the width of the page.
When you place (a) a fixed width sidebar and (b) an element with responsive width to the right/left of this sidebar, and you want these elements to behave properly, the responsive element’s width needs to be calculated in relation to [page width minus sidebar width]. This isn’t possible in actual Muse, and as far as I see, one of the few design options, which can’t be achieved „out of the box“.
If you want a sidebar (and not a "slide in“ or something like a „hamburger style“ menu), the only workaround is, to make the sidebar responsive too. This requires deeper considerations on your page design, but it works.
Many thanks all for the quick responses and suggestions! I definitely want the sidebar menu to stay at a fixed width, so it looks like I'll need play with 3rd party widgets. Thanks again!
Thanks for the link to the pre-release discussion. Interesting read with insight about various subtleties to the program. Will keep playing with it!