on macOS where (currently in a virtual machine) I get GDK_MODIFIER_RESERVED_25_MASK instead of GDK_MOD1_MASK
Just tried outside the VM and there it works correctly and I get `GDK_MOD1_MASK`. So IMO this patch (OK, the documentation too) is really all it's needed to support multiple carets.
Dropping the work-in-progress tag.