I tested in Opera Stable and Opera Developer on Linux Mint XFCE and pinning doesn't work there either. When left-clicking on the pin icon or anywhere on the bar the icon is on, the cursor icon turns to a hand that grabs. In other words, it's as if the icon isn't even there as far as clicking goes. Left-clicking and holding on the bar and dragging is like dragging the title bar to unmaximize the window and move it around.
