Continuously scroll map with two fingers

src/gfx.cpp

@@ -41,6 +41,7 @@ bool _left_button_down;     ///< Is left mouse button pressed?
 bool _left_button_clicked;  ///< Is left mouse button clicked?
 bool _right_button_down;    ///< Is right mouse button pressed?
 bool _right_button_clicked; ///< Is right mouse button clicked?
+Point _right_button_down_pos; ///< Pos of right mouse button click, for drag and drop
 
 DrawPixelInfo _screen;
 bool _screen_disable_anim = false;   ///< Disable palette animation (important for 32bpp-anim blitter during giant screenshot)

src/gfx_func.h

@@ -65,6 +65,7 @@ extern bool _left_button_down;
 extern bool _left_button_clicked;
 extern bool _right_button_down;
 extern bool _right_button_clicked;
+extern Point _right_button_down_pos;
 
 extern DrawPixelInfo _screen;
 extern bool _screen_disable_anim;   ///< Disable palette animation (important for 32bpp-anim blitter during giant screenshot)

src/video/sdl_v.cpp

@@ -598,6 +598,8 @@ int VideoDriver_SDL::PollEvent()
 				case SDL_BUTTON_RIGHT:
 					_right_button_down = true;
 					_right_button_clicked = true;
+					_right_button_down_pos.x = ev.motion.x;
+					_right_button_down_pos.y = ev.motion.y;
 					break;
 
 				case SDL_BUTTON_WHEELUP:   _cursor.wheel--; break;

src/window.cpp

@@ -2777,6 +2777,24 @@ static void HandleAutoscroll()
 #undef scrollspeed
 }
 
+/**
+ * Perform small continuous scrolling with right button press and drag.
+ */
+static void HandleContinuousScroll()
+{
+#define scrollspeed 0.05f
+	if (_scrolling_viewport && _right_button_down) {
+		Window *w = FindWindowFromPt(_right_button_down_pos.x, _right_button_down_pos.y);
+		if (w == NULL || w->flags & WF_DISABLE_VP_SCROLL) return;
+		ViewPort *vp = IsPtInWindowViewport(w, _right_button_down_pos.x, _right_button_down_pos.y);
+		if (vp == NULL) return;
+
+		w->viewport->dest_scrollpos_x += ScaleByZoom(scrollspeed * (_right_button_down_pos.x - _cursor.pos.x), vp->zoom);
+		w->viewport->dest_scrollpos_y += ScaleByZoom(scrollspeed * (_right_button_down_pos.y - _cursor.pos.y), vp->zoom);
+	}
+#undef scrollspeed
+}
+
 enum MouseClick {
 	MC_NONE = 0,
 	MC_LEFT,
@@ -3096,6 +3114,7 @@ void InputLoop()
 	/* HandleMouseEvents was already called for this tick */
 	HandleMouseEvents();
 	HandleAutoscroll();
+	HandleContinuousScroll();
 }
 
 /**