PSE 2023 Editor opens full screen as expected but only shows half as workspace (see image)

[GCS-BobK]

As per subject - image attached.
PSE Version 21.0 (20230329.Git.master.77124fa)x64 on Windows 11 Pro 22H2
Only a casual user so probably messed up a setting somewhere but help appreciated.

[GCS-BobK]

Found that since the 'Expert' heading was still visible, clicking that returned the full workspace, since which PSE has opened normally. The issue seems to have occurred after using guided mode to lighten/darken a layerwith the 'before and after horizontal 'display format selected. More investigation needed methinks...

[Glenn 8675309]

Never have seen that, in 14 years of use.