1) Set Settings > Tabs > Tab Layout to Standard.
2) Have several tabs open.
3) Click the Enable Focus Mode button in the URL bar.
4) All the tabs vanish and won't come back until Focus mode is disabled or you switch to a different Tab Layout
I expect the tabs to stay visible and attached to the Toolbar. They should only disappear when the toolbar itself does, i.e. when you move your pointer away from that area. And reappear when you move your pointer back to that area, like the toolbar does.
0.99.133-rc
Sequoia (15)