WL11 Supervision stopped allowing pane splitting

I created a simple 9-pane Supervision window for a monitoring workflow which worked fine.
After starting up again I wanted to add another meter to it but the pane splitting icons are greyed out and won’t allow me to split any of the panes in the window.
Appears to be an issue, unless I have inadvertently set something to prevent alteration of the supervision setup.

Check the doc, but I think the total number of panes could have a limit.

You are right. 9 module slots maximum.