Nutrient
One item: a vibrant/standard M3 icon button. A selected item (the SDK marks the item of an open outline, grid or reader view, and an active tool, as selected) renders with a filled container, so "on" reads as such and invites the tap that turns it off. Only a ToolbarItem.Toggle is a Material toggle button; a ToolbarItem.Button is a plain icon button painted with the same colors, so its merged semantics carry selected and never a checked state. A ToolbarItem.Submenu renders as the same button with a submenu corner mark; tapping it opens the children in a Material 3 dropdown, and a child tap dispatches ToolbarAction.SubmenuChildClicked.
A long press names the button with a Material 3 tooltip, opening away from edge — the window edge the host's pill is docked against — so the chip lands over the document rather than off screen. A group gets none: its long press already opens the dropdown. A button rendered on its own, outside a pill, keeps the default bottom dock and opens its tooltip upwards.