What to look for when testing individual switches
The advantage of a mechanical keyboard when something goes wrong is that the fault is almost always confined to one switch, and one switch is a repairable unit. A membrane keyboard with a dead key is usually a keyboard to replace. A mechanical keyboard with a dead key is a five-minute job on a hot-swap board and a soldering job on everything else. So the goal of testing here is narrower than on other layouts: find the specific switch, and work out which of the handful of switch failures you are looking at.
The panel beside the layout counts presses per key, which is what makes the most common mechanical fault visible. Tap a key once, firmly, and watch its count. If it goes up by two, the switch is chattering — the metal leaf inside is bouncing on contact and the controller is reading each bounce as a keystroke. Every mechanical switch bounces; the firmware normally hides it with a debounce window, typically around five milliseconds. A switch chatters when its bounce outlasts that window, which happens as contacts oxidise, pick up contamination, or lose spring tension with age.
For the actual millisecond figures, the double typing test measures the gap between the two events and tells you whether it is inside or outside a plausible debounce window — useful evidence before you replace anything. If you want to hear what a switch type sounds like before buying, the keyboard sound test plays the common profiles.
Work across the board row by row rather than jumping to the key you suspect. A single misbehaving switch and a whole dead row mean different things: switches fail individually, but a row failing at once points at the controller or a trace, and no amount of switch swapping will fix that.