Vlad a small point — when hovering the divide between the vertical Tab Bar and the page, the cursor swapped to left pointing arrow, implying the divider could only be dragged left.
In fact the divider could not be dragged any further left, and the only useful function of dragging the divider is to reveal the page titles by dragging right.
Hovering the divider top of window, between the Tab Bar and window Toolbar shows the more appropriate left/right arrow cursor, but again, I can't drag any further left so in theory at this point the arrow should be pointing right, but I think the left/right arrow is macOS convention.
Observe in Safari, while mouse-down, once I've dragged the max distance left (to collapse the Tab Group sidebar) the cursor swaps to the right arrow variant, suggesting that's the only way to drag at this point.
That could help a little, but it doesn't solve for discoverability — there's no current signifier to draw one into hovering the divider in the first place, but this is true of this convention on macOS in general.