Calltip is currently displayed when typing the opening parenthesis behind the function name. When the source code is finished Ctrl+Space between the parentheses brings up the tooltip for the function too. MSVS shows the tooltip on one more occasion - when the mouse cursor is hovering above the function name. If can useful when reading some code in order to understand it and to inspect the functions or properties just sweep the mouse above them.
I hooked to SCN_DWELLSTART and SCN_DWELLEND to show and hide the calltip. What do you think?