Hello,
my name is Raymond.
I have a patch for you, with a reimplentation of IsReactOS() in reactos/sdk/include/psdk/versionhelpers.h.
This function was marked FIXME.
The old check with installation directory is error prone.
My implementation read the field "ProductName" of VersionsInformation resource from shell32.dll.
A second problem, I have found in reactos/dll/win32/comdlg32/filedlg.c.
Duplicates in the function filedlg_collect_places_pidls().
My resolve used a little function collection in the files cstrlist.c and cstrlist.h.
In filedlg-snipped.c is the new code.
I don't know if this approach is desirable and where such helper functions should be placed.
So decide for yourself about their use.
I hope this little step is a good begin.
Greetings
Raymond