Using computedStyle instead of style when reading `top` and `left` from element

[willnew]

We often set property inside css file instead of inline, so I suggest using computed style instead of style when reading top and left, the code here might explain what I mean

https://github.com/willnew/angular-drop/commit/5eb3c8de09f2cec3c540b376df087b4985b473c3

[willnew]

And It's just work and further changes are needed if you think it is necessary to make that change

[caitp]

care to send a PR?

[willnew]

I'll see