I think it is FF where you can put the mouse pointer in the row of tabs and scroll them horizontally (with the mouse-wheel). It slides the list, but does not change the tab in focus. You can scroll a tab to the mouse and click it without moving the mouse to the tab. Would that be useful with Brave? I guess it changes the pane functions, but maybe the concepts could be combined:
The pane indicator row and the tab row could be rendered scrollable (as per above, and again to view the tabs ahead of click-selecting). I recommend that “course increments” be used rather than fixed panes. It would really be a course scroll in the “panes” row, paired with a fine scroll in the tab row. If you want to view a group of tabs that span the 20th tab (any border between panes), you would not have to toggle between panes to see that set. Would others find this useful?