In the current implementation, the candidate window position of a input
method is fixed because XNSpotLocation isn't updated after an input
context (XIC) is created in X11DRV_CreateIC().
X11DRV_UpdateCandidatePos() in this patch updates the position. You can
see the change of a position with ibus, scim or fcitx when input style
is set to "over the spot" in the registry key:

[HKEY_CURRENT_USER\Software\Wine\X11 Driver]
"InputStyle"="OverTheSpot"

This patch was based on the original work by Muneyuki Noguchi, and
received a lot of help from Sebastian Lackner.