Two metal leaves meeting do not make one clean contact. They bounce — closing, opening and closing again several times over a few milliseconds — before settling. Left alone, a single press would register as four or five clicks.
Every mouse solves this the same way: after the first contact, the controller ignores further state changes for a fixed window, typically 1 to 8 milliseconds. That window is the debounce guard.
The cost of the guard
Debounce sets a floor on how close together two accepted presses can be. An 8 ms guard caps the mouse at 125 clicks per second — far above any hand, so it is never the limit in practice. What it does cost is latency: every press is reported after the guard, not at the instant of contact.
This is why gaming mice advertise low debounce times, and why setting one too low is a real risk rather than a free win.
When the guard stops working
Switches wear. The contacts pit, the bounce gets longer and louder, and eventually it outlasts the guard. The symptom is unmistakable once you know it:
- Your click rate improves for no reason. A reading 100 CPM above your normal, without a change in technique, is chatter being counted as clicks.
- Single clicks register as doubles in ordinary use — dragging a file opens it, a single click on a link fires twice.
- The per-second profile shows spikes — one bar two or three times the height of its neighbours, where a burst was recorded.
What to do about it
Nothing in software fixes a worn switch; the mouse is reporting exactly what it is being told. Replace the switch, or replace the mouse. Until then, treat every reading from it as invalid — including the flattering ones, especially the flattering ones.