AFAIK nanovg is solely vector graphics and doesn’t deal with input.
Not an expert here, just on the same learning curve, but I think what you’re looking for is to override OnHover, OnButton (for mouse down), onDragHover (for captured mouse movement), etc… in your subclass of Widget.
It doesn’t have anything to do with nanovg. I was deriving my class from ParamWidget … and presumably that no longer has this method. In fact I can’t even find ParamWidget now, possibly it’s been renamed / deprecated in favour of something else.
Looks like event-consuming widgets should dervice from OpaqueWidget.
Most of this isn’t hard to figure out if you read the source. The API docs are mainly useful for browsing and the generated diagrams. The actual written documentation present is inadequate - merely a regurgitation of extremely sparse comments.