The bug I noticed regarding the vertical tabs.
enable vertical tabs > adjust the width > clict the collapse button > enable the sidebar (not autohide mode) > click the expand button = the vertical tabs pane won't expand.
*Another thing I noticed is that pinned sidebar = pinned vertical tabs panel. It seems like the vertical tabs is not a part of the sidebar so it should stay the same regardless the sidebar is pinned or not.
*The vertical tabs should have option to place it on the left or right edge (or sidebar could have such option, or both of them) to not overload the left edge.
*In vertical tabs you use the color indication to show the tabs reference to tab island. In horizontal mode such indication is missed inthe both expanded tabs on the tabstrip and in island's dropdown on hover (where is still no highlighting effect on cursor hover)