Modal window: resizable abnormally works on bottom left but not on bottom right corner

Modal window: resizable abnormally works on bottom left but not on bottom right corner

Version: 1.8.10

My jQuery modal window works properly (thanks to you guys for such a beautiful job).

I have one issue though and that is: I want to resize the modal window but it does not let me do it from bottom right corner of window, instead the resizing works on bottom left corner of window, also the image to represent resizing is missing.

Why does this happen and how to fix it?