Well, putting the help files directly into root of the installation isn't very nice, as they will get symlinked into environments this way. Isn't there some easy way of overriding the path that is searched for help files?

While I agree in part, this is the pattern that FreeCAD follows (it's not really FHS compliant). These are the directories created in the current version:

$ ls -l /nix/store/rgg0d1rmc1wb6l8fybsanxpjsf05k5y9-freecad-0.18.2/
total 24
dr-xr-xr-x  2 root root 4096 Jan  1  1970 bin
dr-xr-xr-x  5 root root 4096 Jan  1  1970 data
dr-xr-xr-x  3 root root 4096 Jan  1  1970 Ext
dr-xr-xr-x  2 root root 4096 Jan  1  1970 lib
dr-xr-xr-x 35 root root 4096 Jan  1  1970 Mod
dr-xr-xr-x  7 root root 4096 Jan  1  1970 share

The doc directory would be there too except the fixup phase moves it to $out/share/doc.

Another approach would be to have a subdirectory like $out/freecad that all the resource directories get created in. Happy to look into that (presumably it's possible by just setting the install prefix, but I haven't investigated yet).

Oh, the doc in package root isn't that bad... I somehow thought it would be one level lower.

I should've written I can't really test it, as for me it won't start up at all, regardless of this PR:

failed to create drawable
[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
FreeCAD: xcb_io.c:263: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.

@vcunat Try env LIBGL_DRI3_DISABLE=true FreeCAD --- see here: https://forum.freecadweb.org/viewtopic.php?t=28054 I have the same problem with amdgpu.

FWIW I don't get that error using Intel graphics on Wayland.

Hmm, OK, it was amdgpu for me as well. Intel skylake + X seems fine as it is. Perhaps we should use a wrapper to set this variable? Do you know about any down-sides? (I haven't read that forum thread properly, at least not yet.)