This is an old issue, I just finally got around to reporting it.
SlickEdit Pro 2015 (v20.0.1.3 64-bit)
Build Date: December 20, 2015
Emulation: Vim
OS: Linux
OS Version: Ubuntu 14.04.3 LTS
Kernel Level: 3.19.0-47-generic
Build Version: #53~14.04.1-Ubuntu SMP Mon Jan 18 16:09:14 UTC 2016
Processor Architecture: x86_64
If I have a readonly file open and wish to visually (or horizontally) split the window so that I can look at different parts of the same file at the same time, I get an error pop up "This command is not allowed in Read Only mode." Yet, if I go to the menu Windows->Split Vertically (or Horizontally) it does what is expected. I would expect :vs and :sp in the vi emulation to work the same as from the menu.