(XTread_socket): ButtonPress clears f->mouse_moved.

src/xterm.c

@@ -4566,6 +4566,11 @@ XTread_socket (sd, bufp, numchars, expected)
 		  {
 		    dpyinfo->grabbed |= (1 << event.xbutton.button);
 		    last_mouse_frame = f;
+		    /* Ignore any mouse motion that happened
+		       before this event; any subsequent mouse-movement
+		       Emacs events should reflect only motion after
+		       the ButtonPress.  */
+		    f->mouse_moved = 0;
 		  }
 		else
 		  {