I have no issue docking a custom toolbar in V6. I even saw a user dock a custom toolbar below the left side default toolbars and that worked well for me too once I knew it was possible.
However, when I attempt the same thing in V7 WIP, I don’t get the purple highlight. I get a real-time preview of what the docking will look like, and it either replaced the default toolbar or creates a weird vertical tab.
The purple preview on where the toolbar will dock only appears on built-in toolbars, like the filter toolbar.
It’s also seemingly impossible for me to dock a toolbar to the left of the right side default column.