It is now necessary to use the dropdown menu at the top of the sidebar, choose the "3 dots" menu on the top to see a hierarchical menu to see the allegedly open tabs. Indeed the tabs are there. But, the correct vertical display behavior is broken. The plus to open a new tab no longer appears at the end of the list of tabs in the side bar. Indeed, there is no list of tabs at all.
The only way to fix it is to switch back and forth between horizontal and vertical tabs.
Yet another appearance and feature bug.
The correct behavior is a list of tabs and the ability to move the mouse pointer to the left edge of the browser window and have the sidebar open.
Version 0.99.133-beta (WebKit 621.1.2.111.4)
Sequoia (15)