Had the same prob Sanadee.
I was able to resize and move the window when I changed the position of the tabs to the bottom of the chatbox. When separating the tabs again its still not easy to catch the spot where you can resize/move the window, but it's doable
Try to catch it at the bottom of the window, where the Tab would normally sit, near some border line.
Hope that helps - good luck
edit: added missing word...