Use window.inner(Height|Width) instead of document.body.client(Height|Width)

[Tey]

When the body height/width is smaller than the window size, root_h/root_w will be smaller than the window size, so the winboxes will not be able to be moved around all the available space. Please use window.innerHeight and window.innerWidth to set the value of root_h/root_w in init() instead to prevent that.

Here is an example of the issue: https://jsfiddle.net/pahcfy63/1/ (try moving the winbox down or minimize it)