I discovered, by accident, today, that the Timeline column header in the Network panel is clickable. That's not discoverable or intuitive at all. This should be more obvious.
There is now a bit of a knob at the start of the timeline, which has a cursor state. Video editing software frequently keeps the normal cursor and shows a skinny vertical line over the position in the timeline you are currently hovering over. See: your favorite video editing software.
This UI doesn't exist any more.