I agree, Alan - This is an annoying inconsistency (between PhotoLibrary & Customise modes).
In PhotoLibrary mode, key-board shortcuts are available as follows;
- These shortcuts enable quick & easy control of whether LHS and/or RHS panels appear, or not.
However, when in Customize mode, only the F9 key-board shortcut is available;
You can vote here to have this inconsistency corrected.