Slow Typing notification blocks LMDE XFCE UP5

[on4aa]

Since I have upgraded my LMDE XFCE to UP5, a "Slow Typing" notification with a keyboard icon occasionally appears for about one second after which the computer no longer accepts any keyboard input.
It appears to happen more often when dragging text, but it is very hard to reproduce the error.
I could not find any documentation about any "Slow Typing" feature.
As far as I know, the error is unrecoverable. I have to perform a hard reset, potentially resulting in data loss.

All suggestions are very welcome.

This issue has also been reported as a bug: https://bugs.launchpad.net/linuxmint/+bug/1056395

[on4aa]

"Slow Typing" can be deactivated by pressing a [Shift] key for a very long time.
Incidentally, it can also be turned on in this manner.

Nonetheless, this remains a bug because I have noticed that "Slow Typing" sometimes turns on without holding any key.

Also, this needs to be documented in the help.