It has been observed (in the HTerm terminal emulator) that the
event stored in the 'xterm-mouse-last-down' terminal parameter gets
overwritten during a mouse drag operation, causing Emacs to attempt to
synthesize the non-existing <drag-mouse-0> event. Copy the event into
the terminal parameter to protect against such modifications.
* lisp/xt-mouse.el (xterm-mouse-translate-1): Guard against modification
of input event list.