set
The host's veto over the main toolbar's visibility — the Compose counterpart of the View MainToolbar.setForcedVisibility(GONE) hosts used to keep the SDK's bar hidden for good.
The SDK drives setMainToolbarVisible on every UI-visibility recompute (immersive mode, contextual bars docking on top, the UI-visible toggle), which would undo a plain setMainToolbarVisible(false) from a host on the next recompute. This flag is combined with the SDK's request instead: frame.isVisible is false while it is false, whatever the SDK asks for, and true (the default) hands control back to the SDK's request. Unlike PdfActivityConfiguration.defaultToolbarEnabled(false) it is a runtime toggle and needs no fragment recreation.
Not saved in the state snapshot; the host re-applies it like the rest of its customization.