How this test detects it
Press the suspect key normally, once, at the speed you would type it. The tester watches the gap between releasing a key and it going down again. Anything under forty milliseconds is flagged as chatter and the cap turns amber.
Forty milliseconds is chosen deliberately. Mechanical switch contacts settle within a few milliseconds, and a human double tap is never faster than about eighty. A second press landing inside that window did not come from your finger.
If the key only chatters sometimes, press it twenty or thirty times. Intermittent chatter is the normal early stage and will not show on a single press.
What causes it
Every mechanical contact bounces. When two pieces of metal meet, they make and break contact several times in the first few milliseconds before settling. Keyboard firmware hides this with a debounce routine: after it sees a change, it ignores that key briefly.
Chatter happens when the bouncing outlasts the debounce window. Contamination, oxidation and mechanical wear all lengthen the bounce, so a switch that was fine for years starts producing doubles as it ages. Dust and spilled liquid do it faster.
Membrane keyboards and laptop keyboards chatter too, usually after a spill, and on a laptop the whole assembly is often a single part.
What fixes it, roughly in order of effort
- Compressed air into the switch from a couple of angles, with the keyboard held at an angle so debris falls out rather than further in.
- Contact cleaner made for electronics, applied sparingly, working the switch a few dozen times afterwards. Not a lubricant and not a household cleaner.
- On a hot-swap keyboard, pull the switch and put in another one. This is a two-minute job and it is a definitive fix.
- On a soldered board, desolder and replace the switch, which needs an iron and some care.
- Some firmware, including QMK and VIA, lets you lengthen the debounce time. That masks the symptom and is a legitimate stopgap, at the cost of a little added latency.
When it is not the switch
A wireless keyboard on a weak connection can deliver a repeated key because a packet arrived twice or a release was lost. Try it wired, or with the receiver in a front-panel port, before blaming the hardware.
Windows has an accessibility feature called Filter Keys which changes how repeats are handled. It is designed to suppress doubles rather than create them, but if it has been enabled accidentally it can make a healthy keyboard behave oddly, and it is worth checking that it is off.