Controls (desktop): Left click = open, Right click = flag (or flag → ? → blank if enabled), Middle click or both buttons on a revealed number = chord.
Controls (mobile): Tap = open, Long‑press = flag, Double‑tap a revealed number = chord (opens neighbors if flags match).
First click is guaranteed safe (and will not be a number surrounded by mines only if that’s possible via the normal rule: mines are placed after the first click).