What steps will reproduce the problem?
1. Execute ":setlocal conceallevel=3 concealcursor=nvic |syn match Foo conceal 
'^....................'".
2. Make vim window narrow, say ~40 columns.
3. Type "The quick brown fox jumps over the lazy dog." twice.

What is the expected output? What do you see instead?
Vim should not wrap the line until the visible text actually reaches the edge 
of the window, but it wraps apparently 20 columns before the edge, as if the 
concealed characters still contribute to the effective length.

What version of the product are you using? On what operating system?
vim 7.4.430 on Ubuntu Linux 14.04