I can reproduce this but the keyboard shortcut Ctrl/Cmd+Z works for me.

I suspect fixing the menu item to work is going to be very hard with the current state of CodeMirror.

Lol, i've been hearing about this issue a lot, and never thought to check the menu... I don't think I can remember the last time I used a menu for undo. Shows you how fixed you can be into a pattern that other users might not be into.

Safari too for me (latest version on macOS); the Undo menu item is non-functional when the syntax highlighter (CodeMirror) is on, just like with the other browsers.

This is an known upstream issue, for which there is no workaround, because browsers don't support this level of integration with the pasteboard yet.

https://github.com/codemirror/CodeMirror/issues/4809
https://github.com/codemirror/CodeMirror/blob/master/doc/manual.html#L968

In T200280#5914051, @TheDJ wrote:

This is probably the same as T33780: WikiEditor dialogs kill the undo buffer

No. T33780: WikiEditor dialogs kill the undo buffer happens with disabled CodeMirror. With enabled CodeMirror the undo buffer works correctly when using a WikiEditor button to insert characters.

For some reason, it works (not reproducible) in Internet Explorer 11.