I reported this same issue to the mailing list a week or two ago.
Bram wrote:
I know about this: When you type the "." and there no complete match was inserted (showing the longest common text in this example), Vim assumes you are extending the text to reduce the list of matches. Thus the completion still starts at "BlahBlah". You need to stop completion somehow, e.g., with a space and backspace. This is not a nice way to work. I thought of having all punctuation characters stop completion, but that breaks completion of items where punctuation can be part of the match (e.g., () for functions).
The same bug affects file name completion and presumably others as well: /u<C-X><C-F>/b<C-N> Result with completeopt-=longest /usr/bin/ with '/usr/bin/' as only suggestion. Result with completeopt+=longest /usr/b with no suggestions. -- eric
