When the icompletion-vertical-mode setting is on, looking up a manpage
feels unresponsive, for example, looking up 'awk' echoes 'a' in the
minibuffer at the 'Manual entry:' prompt, then finds
'awk [No matches]' but shows the manpage on RET.

When completion-styles is

> (basic partial-completion emacs22)

# steps to reproduce behavior
* start by 'emacs -Q'
* customize 'icompletion-vertical-mode' to enable for session
* type 'M-x man RET awk'

# observed behavior
# minibuffer at 'Manual entry:' prompt
* shows 'a' of awk and is unresponsive, then 'awk [No matches]' shows
* awk RET shows *Man awk* AWK(1) page

# expected behavior
# minibuffer at 'Manual entry:' prompt
* progressively shows 'a' 'aw' 'awk' while typing
* progressively shows icompletion-vertical-mode suggestions
* awk RET shows *Man awk* AWK(1) page

-- 
vl