The state that the sidebar is always displayed is too inconvenient. The sidebar should be hidden on startup. When calling Collabora internally in Slack etc., even though the display area is small, it is further narrowed down by the sidebar. Also the current sidebar is still incomplete. The background color can't be made transparent. Loolwsd 4.2.1, Collabora 6.2.8
Indeed, the app should remember if the sidebar was visible or not between runs (and in Impress that which sidebar deck was open). Probably the simplest way to do it is via cookies.
This should be fine in recent Collabora Online 6.4 versions, at least the online side to remember last setting (per app) is there. The initial state should require support from the integration, but once the user hides/unhides the sidebar, the setting is saved to the browser's local storage (so, it's per browser, not per user). It's also remembered separately per each app.