Userscripts.org

Why not just prevent the dialog from leaving the screen? This is what I use for movement:

var initial_mouseX
var initial_mouseY
var initial_boxX
var initial_boxY
var el
function startMove (event) {//TODO wrap this up in an object
        el = this.parentNode
        initial_mouseX = event.clientX
        initial_mouseY = event.clientY
        if (el.style.right)
                initial_boxX = 0
        else
                initial_boxX = document.body.clientWidth - el.offsetWidth - parseInt(el.style.left)
        if (el.style.bottom)
                initial_boxY = document.body.clientHeight - el.offsetHeight
        else
                initial_boxY = parseInt(el.style.top)
        document.addEventListener('mousemove', move, true)
        document.addEventListener('mouseup', endMove, true)
}

function move (event) {
        var right = initial_boxX + initial_mouseX - event.clientX
        var left = document.body.clientWidth - el.offsetWidth - right
        el.style.right = ''
        if (right < 15) {
                el.style.right = 0
                el.style.left = ''
        } else if (left < 15)
                el.style.left = 0
        else
                el.style.left = left + 'px'
        var top = initial_boxY - initial_mouseY + event.clientY
        var bottom = document.body.clientHeight - el.offsetHeight - top
        el.style.bottom = ''
        if (bottom < 15) {
                el.style.bottom = 0
                el.style.top = ''
        } else if (top < 15)
                el.style.top = 0
        else
                el.style.top = top + 'px'
}

function endMove () {
        document.removeEventListener('mousemove', move, true)
        document.removeEventListener('mouseup', endMove, true)
        GM_setValue(el.id + '_left', el.style.left)
        GM_setValue(el.id + '_top', el.style.top)
}

When the dialog is within 15 pixels of an edge, it will 'jump' to that edge w/o going past it. I haven't tested it on us.o so you might have to change document.body.clientWidth to document.documentElement.clientWidth or something, since different websites are weird like that.

It just bugs me that your script registers three menu commands.

Edit: bugfix