Hello, gaconvn.

Thank you for contacting us.  Option-clicking the disclosure triangle of an item works the same in BookMacster as in any other Mac app.  To be sure, I just retested this.  All descendant folders are expanded or collapsed.

This leaves the issue of items at the root level, of which there could be many.  To expand or collapse these items, click in the menu: Bookmarks > Expand Roots or > Collapse Roots.

I do see a possible feature request that option Expand Roots or > Collapse Roots would expand or collapse all items in the entire document.  But this would require two more menu items since the option key would become part of the menu item's keyboard shortcut.  Is that what you mean?