MatExpIconButton

The toggle state of the icon button ('selected' | 'unselected').

Inside a MatExpButtonGroup, the group is the source of truth: it manages this model for every projected button in response to clicks, keeping it in sync with the group's single-/multi-select state.

Standalone (outside a group), a click does not flip this value automatically — the directive's click handler only delegates to a parent group when one is present. Toggling a standalone button is consumer-driven: two-way bind toggle and flip it yourself in your own (click) handler, e.g. [(toggle)]="state" (click)="state.set(state() === 'selected' ? 'unselected' : 'selected')". This is an intentional, documented contract (see issue #188) — not a bug.