I have set some options to configure completion on the command line, for commands that support it.
I use the completion on the commandline very often with the
:help command; That will be my example, but my question is about commandline completion in general.
If I have only a vague idea how the help topic, I type a substring I expect in the name, and press tab, showing a list of possible completions.
The list is often long enough to be not easily read through; this is an example where a search in the list would be useful.
In the screen shown below, the substring entered was
CTRL-W. The first Tab listed the four columns. (Note it includes variants not starting with the substring.)
After pressing Tab again, the second to last line was shown, repeating part of the table above it. Further Tab presses cycle through that line.
CTRL-W CTRL-W_b CTRL-W_g] CTRL-W_CTRL-I
CTRL-W_+ CTRL-W_c CTRL-W_g} CTRL-W_CTRL-J
CTRL-W_- CTRL-W_d c_CTRL-W CTRL-W_CTRL-K
CTRL-W_< CTRL-W_f i_CTRL-W CTRL-W_CTRL-L
CTRL-W_= CTRL-W_h CTRL-W_gF CTRL-W_CTRL-N
CTRL-W_> CTRL-W_i CTRL-W_gf CTRL-W_CTRL-O
CTRL-W_] CTRL-W_j CTRL-W_<BS> CTRL-W_CTRL-P
CTRL-W_^ CTRL-W_k CTRL-W_<CR> CTRL-W_CTRL-Q
CTRL-W__ CTRL-W_l CTRL-W_<Up> CTRL-W_CTRL-R
CTRL-W_} CTRL-W_n CTRL-W_bar CTRL-W_CTRL-S
CTRL-W_F CTRL-W_o CTRL-W_<Down> CTRL-W_CTRL-T
CTRL-W_H CTRL-W_p CTRL-W_<Left> CTRL-W_CTRL-V
CTRL-W_J CTRL-W_q CTRL-W_CTRL-] CTRL-W_CTRL-W
CTRL-W_K CTRL-W_r CTRL-W_CTRL-^ CTRL-W_CTRL-X
CTRL-W_L CTRL-W_s CTRL-W_CTRL-_ CTRL-W_CTRL-Z
CTRL-W_P CTRL-W_t CTRL-W_CTRL-B CTRL-W_<Enter>
CTRL-W_R CTRL-W_v CTRL-W_CTRL-C CTRL-W_<Right>
CTRL-W_S CTRL-W_w CTRL-W_CTRL-D CTRL-W_g_CTRL-]
CTRL-W_T CTRL-W_x CTRL-W_CTRL-F c_CTRL-R_CTRL-W
CTRL-W_W CTRL-W_z CTRL-W_CTRL-H
CTRL-W CTRL-W_+ CTRL-W_- CTRL-W_< CTRL-W_= CTRL-W_> CTRL-W_] CTRL-W_^ >
The list of completions is often longer than one page, which causes a prompt for the next page.
In that case, the search would be even more helpful.