When verticals tabs are enabled:
In this case I would like for an option called "Auto Hide sidebar when in fullscreen" or "Always Show Vertical Tabs in Full Screen" (Sidebar here is tabs+bookmarks). This would be similar to "Always show Toolbar when in Full Screen"
Not in full screen:
However if this "Auto Hide vertical tabs" feature isn't enabled, things would look as they have been for so long in Orion(Pre Auto Hide feature addition)
When horizontal tabs are enabled:
In this case, showing sidebar automatically in both full screen and non full screen is futile and should only be accessible by clicking on sidebar button or Cmd+Ctrl+1.
To not have a redundant option for auto hiding the sidebar, I would suggest adding the "Always show Toolbar when in Full Screen" as a subset of the vertical tab setting (That is, this setting is automatically greyed out when horizontal tabs are toggled to avoid confusion (Like in the image below)