Keyboard ghosting explained

Most keyboards scan switches in rows and columns instead of wiring every key to its own circuit. When several keys in one matrix area are held together, the scanner can read an extra intersection as pressed. That phantom result is called ghosting, and the maximum number of simultaneous keys a keyboard can report is its rollover.

Why a test can show it

If you hold three or four keys in one cluster and a fifth key appears in the event history without being touched, the matrix may be reporting a phantom press. The same cluster may also drop one of the real keys, which is the opposite symptom of the same matrix limitation.

Anti-ghosting and NKRO

Keyboards with anti-ghosting add blocking diodes or remap the matrix so common combinations stay accurate. N-key rollover (NKRO) means the keyboard attempts to report every press independently. A browser test can show how many simultaneous presses the keyboard actually sends, but it cannot repair a matrix limitation.

The practical takeaway: if a game combo such as W + A + Shift + Space fails or creates a phantom key, test the same four keys slowly and note exactly which combination starts failing.