If I tap and hold a tab in Opera (version 39.0.2256.48 on Windows 10 Anniversary Update) to bring up the context menu, often the tab will detach from the tab bar. If I quickly notice this has happened I can try to drag it back to the tab bar, but it doesn't let me drop it back onto the bar nor does it show the up-arrow symbol to indicate where the tab would be dropped. Of course, if I don't notice it in time, the tab opens in a new window and I can then try dragging it back to the tab bar. This can be a pain, though, as I usually run Opera maximized.
Perhaps if Opera took a little longer checking for a long-press this wouldn't happen so often.