otclient/modules/old/game_interface/uimap.lua

function UIMap:onDragEnter(mousePos)
  local tile = self:getTile(mousePos)
  if not tile then return false end

  local thing = tile:getTopMoveThing()
  if not thing then return false end

  self.parsed = false
  self.currentDragThing = thing
  Mouse.setTargetCursor()
  return true
end

function UIMap:onDragLeave(droppedWidget, mousePos)
  if not self.parsed then
    self.currentDragThing = nil
  end

  Mouse.restoreCursor()
  return true
end

function UIMap:onDrop(widget, mousePos)
  if not widget or not widget.currentDragThing then return false end

  local tile = self:getTile(mousePos)
  if not tile then return false end

  local count = widget.currentDragThing:getCount()
  if widget.currentDragThing:isStackable() and count > 1 then
    widget.parsed = true
    local moveWindow = displayUI('/game/movewindow.otui')
    local spinbox = moveWindow:getChildById('spinbox')
    spinbox:setMaximum(count)
    spinbox:setMinimum(1)
    spinbox:setCurrentIndex(count)

    local okButton = moveWindow:getChildById('buttonOk')
    okButton.onClick = function()
                         g_game.move(widget.currentDragThing, tile:getPosition(), spinbox:getCurrentIndex())
                         okButton:getParent():destroy()
                         widget.currentDragThing = nil
                       end
    moveWindow.onEnter = okButton.onClick
  else
    g_game.move(widget.currentDragThing, tile:getPosition(), 1)
  end

  return true
end

function UIMap:onMouseRelease(mousePosition, mouseButton)
  local tile = self:getTile(mousePosition)
  if tile and g_game.processMouseAction(mousePosition, mouseButton, nil, tile:getTopLookThing(), tile:getTopUseThing(), tile:getTopCreature(), tile:getTopMultiUseThing()) then return true end
  return false
end