To remove trailing whitespace:

1. Enter PictureMode – `M-x picture-mode
2. Exit picture mode – ‘C-c C-c’
That’s it. The only side effect of this is that all trailing whitespace is removed. I discovered this odd (and undocumented?) “feature” by accident about 20 years ago. So, it has been around since about version 18 of GnuEmacs – perhaps it’s even older. – DrewAdams