It's the main application window. Attached a screenshot of the entire app window (again, 1200x1024 minus the titlebar, which seems to be the default size). This is after setting the Languages/File Extension Manager/Encoding to "UTF8", and restarting SE. Of course, the window was much larger than that when I exited.
Setting it back to "UTF8, no signature", and then adjusting window size will correctly restore it after exit/restart.
SlickEdit Pro 2017 (v22.0.0.2 64-bit)
Serial number: FE11862_BETA
License type: Beta License
License expiration: 2017-11-06 19:00:00
License file: C:\ProgramData\slickedit\22\slickedit.lic
Build Date: August 11, 2017
Emulation: CUA
OS: Windows 10 x64
OS Version: 10.00.0
Memory: 35% Load, 5831MB/16322MB Physical, 7594MB/32706MB Page File, 481MB/134217727MB Virtual
Shell Information: C:\WINDOWS\system32\cmd.exe /q
Screen Size: 2560 x 1600, 1200 x 1600, 1200 x 1600, 1920 x 1080
Project Type: (Other)
Language: .less (Less)
Encoding: Automatic
Installation Directory: C:\SlickEdit Pro 22.0.0 Beta2\ (non-removable drive,NTFS,174563MB free)
Configuration Directory: C:\SlickEdit Pro 22.0.0 Beta2\config\22.0.0\ (non-removable drive,NTFS,174563MB free)