In several places in Piwik's Web interface, pressing the backspace key will delete two characters instead of one, if by doing so it can delete a space at the end of the input.
Steps to reproduce:
I'm running Piwik 3.1.1, server is Linux, Apache, PHP 5.6.31; client is Linux, Pale Moon. It happens to me in every text field in the interface that has fancy fade-in and similar features, throughout the settings and configuration screens, but not in the "search" box and a few other places.
I can reproduce this in Firefox 56.
I feel like this is kind of intended by some library, but I agree with you that it is annoying as the user doesn't expect it.
Sorry for the first commit which was not branched from 3.x-dev