@scriptum: I uploaded a new tag file to the Geany Wiki so maybe you want to try that. Simple auto-completion worked for me using that file.
The file you mentioned seems to have a flaw. That auto-completion is not working with that file has IMHO the following reasons: - it is written in the pipe format - the pipe format automatically imports all tags as function prototypes (```tm_tag_prototype_t```), see https://github.com/geany/geany/blob/master/src/tagmanager/tm_source_file.c#L... - but the tag-manager-parser only allows struct, class and variable tags for CSS, see https://github.com/geany/geany/blob/master/src/tagmanager/tm_parser.c#L171 (didn't check this point in detail, just an informed assumption)
IMHO the issue here is caused by the tags file you used, not Geany itself.