You might be interested in
this macro. It strips trailing whitespace only from modified lines on save.
SlickEdit tends to leave trailing whitespace on edited lines (an example is if you split a line just after whitespace), and it doesn't have a way to highlight trailing whitespace. This appears to be because a lot of people work in small teams with code bases that are fully under their control, and configure their editors to always trim trailing whitespace. Of course that is extremely problematic in large teams or diverse code bases, but the only other option in SlickEdit is to never strip, but since the only way SlickEdit manages trailing whitespace is unilaterally at save via an option (but not dynamically while editing) it ends up meaning that SlickEdit adds to the trailing whitespace pollution.
Sidebar...
Another editor I use has _before_command_ and _after_command_ hooks, which enabled me to write a macro in that editor to dynamically strip trailing whitespace as I edit, and only in the lines I've edited.
SlickEdit does not have these hooks, so the closest thing I was able to achieve in SlickEdit was the macro above. It's not ideal, but it is a good enough solution that I don't whine much about it.
Those two hooks sure would be handy, though. The first thing I would do is port a macro (written by me, no copyright issues) that automatically fixes the indentation of the current line after the cursor moves off of it, if the line was edited.