Device diagnostics research
Why Some Key Combinations Do Nothing
A keyboard is not a bank of independent switches. It is a grid of rows and columns, and the controller scans that grid to decide which keys are down. When three pressed keys form a rectangle on the grid, the fourth corner can appear pressed too. That is ghosting, and it happens in the keyboard before any software is involved. Our evidence file records the mechanism and the rollover definitions that describe it.
What this guide helps you decide
Pressing three keys sometimes registers only two. Is that the browser, the operating system, or the keyboard itself?
This research explains the keyboard matrix mechanism behind ghosting and blocking, and the reason a browser cannot detect the cause. It does not report a measurement for any specific keyboard model.
Try it on the anchor product: BeanNest Tools /device/.
The row-and-column matrix model
The definition set models a keyboard as switches at row and column intersections scanned by a controller. Pressing a switch connects one row to one column, so the controller infers key state from the electrical matrix rather than reading an independent wire for every key.
Why a rectangle creates ambiguity
The recorded ghosting mechanism is three pressed keys forming a rectangle that makes the fourth corner appear pressed. The same matrix ambiguity is also why a third key can be blocked on a two-key-rollover design. Both effects occur before the browser receives keyboard events.
Rollover terms describe different limits
The evidence distinguishes 2KRO, 6KRO, NKRO, and anti-ghosting. Two-key rollover guarantees at most two simultaneous keys; USB boot protocol represents up to six simultaneous non-modifier keys plus modifiers; NKRO uses a custom HID report descriptor; and anti-ghosting is a vendor term that is not standardised and does not imply NKRO.
Diodes address the matrix path, not the browser
The definition set records blocking diodes as the mechanism behind the anti-ghosting claim: they prevent an unintended current path through the matrix. That is a hardware property. It does not give a normal web page visibility into the physical switch state or the matrix scan.
What the browser actually receives
A browser receives keydown and keyup after the keyboard controller and operating system have already produced a key state. The page cannot see the matrix and cannot distinguish a key that was physically pressed but blocked from a key that was never pressed. A browser test can report only the events it actually observed.
How to phrase a browser keyboard result
A supportable result names the keys that produced events, their order, and their event timing. It should not diagnose a matrix fault, claim a specific rollover capability for unmeasured hardware, or infer a physical press that never reached the page. Those stronger conclusions require evidence outside this browser event stream.
First-party evidence and provenance
How we checked this
We derived the mechanism set from documented protocol limits, Web platform scheduling behaviour, and the electrical model of a keyboard matrix, then encoded each constant with its reason in a generator script in this repository. The definitions file is regenerated by running that script. For this guide we derived the keyboard matrix scanning model and the rollover definitions from the electrical and protocol constraints, then recorded why a browser cannot observe the matrix directly.
- Method
- We derived the mechanism set from documented protocol limits, Web platform scheduling behaviour, and the electrical model of a keyboard matrix, then encoded each constant with its reason in a generator script in this repository. The definitions file is regenerated by running that script.
- Environment
- Node.js on a desktop workstation. The evidence is a definition set with recorded reasons, not a hardware measurement on a specific device.
- Captured
- Reviewed by
- BeanNest Studio
Repository evidence artifacts:
tools/research-evidence/device/definitions-2026-09-20.jsontools/research-evidence/device/README.md