When is ScriptableSingleClickTool.OnHoverHitTest called?

I tried overriding OnHoverHitTest in my scriptable tool, but it never gets called. How is the event triggered?

Full context:
I’m creating a scriptable tool to paint objects on a variable grid in my scene. I’ve overridden TestifHitByClick to respond to mouse events & spawn in the object. I’d like to draw a box on the grid cell containing the mouse whenever the mouse moves, but I haven’t found a way to get the input device ray or something similar on each tick or mouse movement in the editor.