Well, I will try to make it work, however the indended road was something like

	virtual bool Pen(Point p, const PenInfo& pn, dword keyflags) {
		if(keyflags&K_SHIFT)
			return false;
		if(pn.pressure) {
			if((!!pn.pressure == !!pen.pressure) && drawing.GetCount())
				drawing.Top().Add(MakeTuple(pn.pressure, p));
			else
				drawing.Add().Add(MakeTuple(pn.pressure, p));
		}
		pen = pn;
		Refresh();
		return true;
	}
	
	void LeftDown(Point p, dword keyflags) override {
		if(keyflags & K_SHIFT) {
			RectTracker tracker(*this);
			tracker.MinSize(Size(-100000,-100000));
			tracker.Track(Rect(p,p));
		}
	}