Abort drag-and-drop from a busy program

In fact, it is possible to do this (at least with recent versions of X).

You can do this by pressing the XF86Ungrab key, introduced in this commit.

By default, this keysym is not bound to any physical key or key combination (it was disabled in 2012 after it was reported as a security hole, since it allows anyone to bypass lock screens by breaking their grabs). On some other setups it is bound to a shortcut that uses the number pad. Obviously, that won't work on laptops that lack a keypad.

But you can still activate the keysym using xdotool:

setxkbmap -option grab:break_actions
xdotool key XF86Ungrab

This works with X.org version 2:1.11.4-0ubuntu10.8 on Ubuntu Precise.