OptionalfullscreenWhen set (via Toolbar fullscreenTargetRef), fullscreen targets this container (e.g. sefer + toolbar).
Locale for UI strings (toolbar, download menu). Defaults to en-US.
"Of" value from the book (for page indicator). Sourced from FlipBook ref.
Ref used by openDownloadMenu command (Ctrl+S) to open the download dropdown. DownloadDropdown sets this.
Optionalpage
Ref set by DownloadDropdown: (from?, to?) => void to run download (entire book or range). Used by download command.