input-method-highlight-flag

input-method-highlight-flag is a variable defined in `mule-cmds.el'.
Its value is

t

Documentation:
If this flag is non-nil, input methods highlight partially-entered text.
For instance, while you are in the middle of a Quail input method sequence,
the text inserted so far is temporarily underlined.
The underlining goes away when you finish or abort the input method sequence.
See also the variable `input-method-verbose-flag'.

You can customize this variable.