Window outside window

When a CSD application opens maximized and I restore it (or unmaximize it), I get a window artifact outside the CSD window (and its client-side shadow) like this:

Affected applications: epiphany, chromium, gnome-contacts, gnome-calendar, some other CSD applications. SSD (ie decorated by window manager) applications are not affected.
Custom settings that may be related: undecorate maximized windows in MATE tweak
Note that the outer decorations only appear when the application starts maximized. They’re not present if the application starts without being maximized.
How do I fix this?

Are you using gtk3-classic?

Have you tried a different window theme?

How is this related? Does changing that setting make a difference to the issue?

No. I used it in KDE in the past, but not in MATE.

Yes, I’ve tried several metacity themes, none of which work as intended.

Yes, when I restart MATE with this option disabled, I don’t have the issue (tested with epiphany). However, I want undecorated maximized windows and I don’t get that either. Apparently it does make a difference, I found this after searching a bit more, should have searched for maximus