I'm wondering if there is any way to get a fixed-width menubar on the panel. I have the icon-only task manager to the right of the menubar, and whenever the menubar width changes, the task manager changes position. This makes it pretty brutal to use, since one expects the task manager to stay in a fixed position.

If the issue with fixed width menubar is that menus may extend beyond the fixed width, why not, in that case, just have a button which allows access to menus that have fallen off the menubar? It would be nice to have an option for fixed-width in any case.

what's the menu bar?

It's the plasmoid that puts window "global menus" in the panel. It's literally called "Window menubar" when adding a plasmoid.

I don't have it but now remember what it is - suggest a bug report/wish on bugs.ke.org would be appropriate

the widget doesn't ship with my distro, though I did find a package

if you use "button form factor" in settings you will get a fixed width but you would need to click on it to access the top line menus

Yes, I'm aware of that, but it's not the way I'd like to use it. I'll file a bug report.