Slider: made slider positioner snap to values on drag

This isn't perfect, since it's very easy to move the mouse faster than the positioner is updated.
However, this is more a bug in GUI2 than the sliders.

src/gui/widgets/scrollbar.hpp

@@ -166,6 +166,11 @@ public:
 		recalculate();
 	}
 
+	float get_pixels_per_step() const
+	{
+		return pixels_per_step_;
+	}
+
 protected:
 	unsigned get_positioner_offset() const
 	{
@@ -177,6 +182,11 @@ protected:
 		return positioner_length_;
 	}
 
+	tpoint get_mouse_position_last_move() const
+	{
+		return mouse_;
+	}
+
 	/**
 	 * See @ref tcontrol::update_canvas.
 	 *
@@ -346,7 +356,7 @@ private:
 	 * @param distance           The distance moved, negative to begin, positive
 	 *                           to end.
 	*/
-	void move_positioner(const int distance);
+	virtual void move_positioner(const int distance);
 
 	/** Inherited from tcontrol. */
 	void load_config_extra() override;

src/gui/widgets/slider.cpp

@@ -44,9 +44,12 @@ tslider::tslider()
 	, minimum_value_label_()
 	, maximum_value_label_()
 	, value_labels_()
+	, current_item_mouse_position_(0,0)
 {
 	connect_signal<event::SDL_KEY_DOWN>(std::bind(
 			&tslider::signal_handler_sdl_key_down, this, _2, _3, _5));
+	connect_signal<event::LEFT_BUTTON_DOWN>(
+			std::bind(&tslider::signal_handler_left_button_down, this, _2, _3));
 	connect_signal<event::LEFT_BUTTON_UP>(
 			std::bind(&tslider::signal_handler_left_button_up, this, _2, _3));
 }
@@ -221,6 +224,35 @@ bool tslider::in_orthogonal_range(const tpoint& coordinate) const
 	return static_cast<size_t>(coordinate.x) < (get_width() - offset_after());
 }
 
+void tslider::update_current_item_mouse_position()
+{
+	tpoint mouse = get_mouse_position();
+	mouse.x -= get_x();
+	mouse.y -= get_y();
+
+	current_item_mouse_position_ = mouse;
+}
+
+void tslider::move_positioner(const int)
+{
+	const int distance_from_last_item = get_length_difference(current_item_mouse_position_, get_mouse_position_last_move());
+
+	if(std::abs(distance_from_last_item) >= get_pixels_per_step()) {
+		const int steps_traveled = distance_from_last_item / get_pixels_per_step();
+		set_item_position(get_item_position() + steps_traveled);
+
+		update_current_item_mouse_position();
+
+		child_callback_positioner_moved();
+
+		fire(event::NOTIFY_MODIFIED, *this, nullptr);
+
+		// positioner_moved_notifier_.notify();
+
+		update_canvas();
+	}
+}
+
 void tslider::update_canvas()
 {
 
@@ -273,6 +305,15 @@ void tslider::signal_handler_sdl_key_down(const event::tevent event,
 	}
 }
 
+void tslider::signal_handler_left_button_down(const event::tevent event, bool& handled)
+{
+	DBG_GUI_E << LOG_HEADER << ' ' << event << ".\n";
+
+	update_current_item_mouse_position();
+
+	handled = true;
+}
+
 void tslider::signal_handler_left_button_up(const event::tevent event,
 											bool& handled)
 {

src/gui/widgets/slider.hpp

@@ -152,6 +152,9 @@ private:
 		return current.x - original.x;
 	}
 
+	/** Inherited from tscrollbar. */
+	void move_positioner(const int distance) override;
+
 	/** See @ref tcontrol::update_canvas. */
 	virtual void update_canvas() override;
 
@@ -175,6 +178,15 @@ private:
 	 */
 	tlabel_creator value_labels_;
 
+	/**
+	 * When initially pessing the positioner and every time a new value is chosen through dragging,
+	 * this value is upda with the mouse position at the time. This allows the widget to track
+	 * how far the mouse has moved since setting the last value.
+	 */
+	tpoint current_item_mouse_position_;
+
+	void update_current_item_mouse_position();
+
 	/** See @ref tcontrol::get_control_type. */
 	virtual const std::string& get_control_type() const override;
 
@@ -191,6 +203,8 @@ private:
 									 bool& handled,
 									 const SDL_Keycode key);
 
+	void signal_handler_left_button_down(const event::tevent event, bool& handled);
+
 	// In this subclass, only used to grab keyboard focus -
 	// see tscrollbar class for more handling of this event.
 	void signal_handler_left_button_up(const event::tevent event,