b:undo_ftplugin cannot undo window-local options in every window showing a buffer #21243

Steps to reproduce

Start Vim:

vim --clean

Then run:

:filetype plugin on
:setglobal colorcolumn=
:set filetype=fixture
:setlocal colorcolumn=73
:let b:undo_ftplugin = 'setlocal colorcolumn<'
:split
:set filetype=text
:windo echo win_getid() (empty(&l:colorcolumn) ? '<empty>' : &l:colorcolumn)
:messages

Actual behaviour

The current window has an empty colorcolumn, but the other window still has colorcolumn=73:

1001 <empty>
1000 73

Expected behaviour

Both windows should stop showing the filetype-specific color column when their shared buffer changes filetype. Generally, :help undo_ftplugin says that "the effect of the previous filetype should be undone" - and this isn't really true.

Obviously, we know that a buffer-scoped undo hook cannot fully undo window-local effects when the buffer is displayed in multiple windows. Just wanted to canonicalize this issue here as I found it today.

Note that this is distinct from #17113.

Version of Vim

9.1.1752

Environment

macOS arm64 + nix