show a popup/tooltip when hovering over a text annotation (c/c++ sdk)

I agree with Test Screen, the simplest solution is to just draw on the screen.  I've used this method many times. Put the drawing code in screen redraw event to create a stable display, or  not if it's just a quick popup. 

Using actual windows adds more complexity than is necessary. 

I'd be inclined to implement the pop-up display simply using platform methods - write to the screen. When the info is no longer needed, cause a screen refresh by invalidating the affected area. If you can do the display by creating a floating window, all the redisplay should be taken care of when you remove it.