边缘吸附：拖拽窗口的贴边停靠

-- 错误：松手随缘，半截出屏找不回
function onDrop(win, x, y)
    win:setPosition(x, y)
end

local SNAP = 40
local function onDrop(win)
    local x, y = win:getPosition()
    local w = win:getContentSize().width
    if x < SNAP then
        x = -w + 30
    elseif x > screenW - SNAP then
        x = screenW - 30
    end
    if y < 0 then y = 0 end
    if y > screenH - w then y = screenH - w end
    win:setPosition(x, y)
end