The overlay itself

Everything it can do.

What the keystroke overlay actually draws, where it sits, how it looks, and what to check when it does not appear at all.

What it draws

How much of your typing reaches the screen, and what the overlay refuses to draw at all.

How it looks

Nine theme families, three keycap styles, and per-category control over every key on screen.

Where it sits

Placement is the setting people change most, and the one that decides whether the overlay helps or covers the thing you are pointing at.

When nothing appears

Usually a permission, sometimes macOS deliberately withholding the keys. Both have a definite answer.