Like described in this thread for Opera Windows, I am seeing this very same issue since last Opera update on my Mac OS.
When firing up my Dev tools I usually work in a separate window. When trying to move the Dev Tools window, window stays static and can no longer be moved anywhere. I am however able to resize it.
Please fix this Or is this a new feature.. if so: I hate this
Thanks for reading
P.S.: running Opera for Mac v. 62.0.3331.18 on Mac OS Mojave v. 10.14.4