That's odd, I don't see that here (I'm also using Fedora 41). Can you start DCP-o-matic in the terminal (just run dcpomatic2) and see if there are any messages printed there?

There is nothing interesting printed:

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content width -26 (allocation 8, extents 17x17) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content height -2 (allocation 8, extents 5x5) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content width -2 (allocation 0, extents 1x1) while allocating gadget (node header, owner GtkNotebook)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:14.201: Negative content height -9 (allocation 1, extents 5x5) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:14.258: Negative content width -33 (allocation 1, extents 17x17) while allocating gadget (node button, owner GtkButton)

First I thought that it might have something to do with my insanely old window manager (I use fvwm-1.24r which is 30 years old, don't ask .. :-)
But I can reproduce the problem with a new account on a clean/new machine under Gnome (both wayland and X) and Plasma.
So it doesn't not seem related to my personal setup.

One more datapoint: when started under the LXDE environment, things are OK (proper minus signs)

And under an Xfce environment things are OK too.

DOM 2.17.28 has the same problem BTW

And DOM 2.18.0 has it too.

Can you run

gsettings get org.gnome.desktop.interface gtk-theme

from a terminal, and if it responds 'Adwaita' please can you attach the file

/usr/share/icons/Adwaita/symbolic/actions/list-remove-symbolic.svg

here? It should be the SVG of the minus icon in the spin button.

The output is always the same (checked again with DOM 2.18.1):

domtest@computer:~$ gsettings get org.gnome.desktop.interface gtk-theme
'Adwaita'

domtest@computer:~$ ls -l /usr/share/icons/Adwaita/symbolic/actions/list-remove-symbolic.svg
-rw-r--r--. 1 root root 194 Sep 16 02:00 /usr/share/icons/Adwaita/symbolic/actions/list-remove-symbolic.svg

(list-remove-symbolic.svg is attached, it is the correct minus icon)

I checked again with several desktop environments:

"GNOME"
"GNOME Classic"
"GNOME Classic on Xorg"
"GNOME on Xorg"
"Openbox"
"Plasma"
==> Wrong minus icon (a cross)

"LXDE"
"Xfce Session"
==> Correct minus icon

I was not allowed to upload a .svg file by Mantis, so I included a screenshot from a file manager

list-remove-symbolic.png (6,491 bytes)   

list-remove-symbolic.png (6,491 bytes)   

Thanks! This is very confusing. Maybe you could do

strace dcpomatic2 2>&1 | grep list-remove-symbolic

and post the output?

domtest@computer:~$ strace dcpomatic2 2>&1 | grep list-remove-symbolic
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 18
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 28
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 29
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 29
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 34
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 34
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 34
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 34
openat(AT_FDCWD, "/usr/share/icons/breeze/actions/16/list-remove-symbolic.svg", O_RDONLY|O_CLOEXEC) = 34
domtest@computer:~$ ls -l /usr/share/icons/breeze/actions/16/list-remove-symbolic.svg
lrwxrwxrwx. 1 root root 15 Dec 14 01:00 /usr/share/icons/breeze/actions/16/list-remove-symbolic.svg -> list-remove.svg
domtest@computer:~$ ls -l /usr/share/icons/breeze/actions/16/list-remove.svg
lrwxrwxrwx. 1 root root 22 Dec 14 01:00 /usr/share/icons/breeze/actions/16/list-remove.svg -> edit-delete-remove.svg
domtest@computer:~$ ls -l /usr/share/icons/breeze/actions/16/edit-delete-remove.svg
lrwxrwxrwx. 1 root root 14 Dec 14 01:00 /usr/share/icons/breeze/actions/16/edit-delete-remove.svg -> paint-none.svg
domtest@computer:~$ ls -l /usr/share/icons/breeze/actions/16/paint-none.svg
-rw-r--r--. 1 root root 526 Dec 6 12:49 /usr/share/icons/breeze/actions/16/paint-none.svg

And this "paint-none.svg" is indeed a cross !

paint-none.png (5,581 bytes)   

paint-none.png (5,581 bytes)   

Looks like this is a bug in GTK for which a fix has been merged.

https://gitlab.gnome.org/GNOME/gtk/-/issues/7080

So I think we just have to wait for this fix to make it into Fedora. I don't think it's easy to fix on the DCP-o-matic end.

You could hack around it by replacing that symlink to list-remove-symbolic.svg with some more appropriate icon.