However, on upgrading to xfce4-panel-4.18.1-1, the icons in the Workspace Switcher disappear for no obvious reason. Downgrading to xfce4-panel-4.18.0-1 makes this issue go away.
I am using EndeavourOS, with xfce4-panel from the Arch Linux extra database; my window manager is XMonad, but otherwise the rest of my desktop uses Xfce.
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I guess you are using window scaling? The problem is actually specific to 4.18.1, which uses a fix for #681 (closed) that is not on master (so not in 4.19.0), and which introduces this regression. Maybe this can be fixed by using the new WnckHandle API of Libwnck. I'll look into it.
Also note that the icons reappear if you increase the panel size above 35. It's probably best to set it to 36 in this case, to have bigger icons in the tasklist.
I am indeed! 2× window scaling, in fact — I have a HiDPI laptop connected to two other monitors. (And then those other monitors are themselves scaled by 2× in the ‘Display’ settings, though that should be irrelevant, I think.)
Also note that the icons reappear if you increase the panel size above 35. It's probably best to set it to 36 in this case, to have bigger icons in the tasklist.
Alas, I actually can’t set it that high. Due to the large horizontal width resulting from three monitors next to each other, the workspace switcher takes up a very large amount of space (as seen on the screenshot) — so if I increase the height too much, the width proportionally expands so that it overlaps the other components of the panel. Adding another row to the switcher resolves that particular problem, but then each individual workspace gets shrunk so much that there’s no space for the icons in the first place!