The mouse landing zone in the gutter between document panes needs to be in the same colour as the mouse landing zone in the gutter between the document panes and the sidebar.
This is especially true in the "Default theme" where the sidebar gutter is dark (matching the sidebar title) but the pane gutter is light (matching the document border).
In the "Light theme", the sidebar gutter has a marginally darker grey for the landing zone, and the pane gutter does not.
Pane gutter colour (v9.4)
Moderators: AmigoJack, helios, bbadmin, Bob Hansen, MudGuard