Before you start
Write down the exact combination that fails, in the order you press it. "Ghosting" reported without a specific key set is untestable, and the order matters: many boards drop the newest key rather than the oldest, so W+A+Space can behave differently from Space+A+W.
Replace any system-reserved keys in that combination with ordinary letters for the first run. If your failing combination includes a Windows key, Command, Alt+Tab or a function key, you cannot tell interception from rollover until the reserved key is out of the picture.
Note whether the board advertises N-key rollover, 6-key rollover, or nothing at all. This sets your expectation: a board with no stated rollover specification dropping a fourth simultaneous key is behaving exactly as built.
Run the check
Open the keyboard test and hold keys one at a time, adding rather than replacing. Press and hold the first, then the second while still holding, then the third, then the fourth. The tool records event.code for every held position and tracks the largest simultaneous set for the run, so you get a number rather than an impression.
Watch what happens at the moment of failure. Two outcomes mean different things: if the newest key never appears, the board or the USB report has run out of slots. If an already-held key vanishes when a new one arrives, that is a matrix collision, and the pair of keys involved is diagnostic.
Repeat the same count of keys in a different region of the board. Matrix collisions depend on physical position, so four keys in the QWERTY row and four keys spread across the board can produce completely different results on the same keyboard.
Finally, verify that every released key clears. A position that stays marked after release is a stuck-key problem, not a rollover problem, and it needs the three-second stuck timer rather than this test.
Read your result
If the limit is a consistent number regardless of which keys you choose — typically six plus modifiers — you are seeing the standard USB HID boot-protocol report limit. Modifiers usually do not count toward it, which is why Ctrl+Shift+Alt plus three letters often works when six plain letters does not.
If the limit varies with position, and specific pairs cancel each other, that is genuine matrix ghosting. Cheaper boards omit the per-key diodes that prevent it, so current finds an unintended path through the matrix and the controller reads a key that was never pressed, or misses one that was.
If a combination fails only when it contains a system key, nothing about the keyboard is involved. The operating system claimed that shortcut before the browser existed in the chain, and that behaviour is identical on a two-thousand-dollar board.
Measurement limits
A page cannot read the keyboard matrix, count diodes or inspect the USB descriptor. It observes which key events arrive, which is enough to demonstrate that a key was dropped but not enough to explain the electrical reason.
The operating system can reserve combinations at a level no browser reaches. Ctrl+Alt+Delete, Cmd+Tab, Alt+Tab and PrintScreen commonly never arrive, and their absence carries no information about the hardware.
Some gaming keyboards switch rollover behaviour depending on a hardware mode toggle or the vendor software's current profile. A result collected in one mode does not describe the other.
What to do next
A repeatable drop in ordinary letter combinations, in more than one region of the board, is a real rollover limit. If those combinations matter for how you play or work, the fix is a keyboard specified for N-key rollover, not a repair.
A limit that appears only in one game is usually that game's own input handling or a conflicting overlay. Test the same combination in the browser first: if it holds there and fails in the game, the keyboard is not the problem.
A dropped key that appears alongside stuck keys, repeated characters or dead positions is a different fault entirely. Treat that as a hardware or contamination problem and run a full key test rather than pursuing rollover.
Symptom table
| Symptom | Likely layer | Next action |
|---|---|---|
| Fourth key disappears | Matrix rollover limit | Repeat with another physical key group |
| A system shortcut is absent | Operating-system interception | Use ordinary keys for the baseline |
| Released key stays marked | Missing keyup or focus loss | Clear held state and repeat while focused |
Common questions
What is the difference between ghosting and rollover?
Ghosting strictly means a key registering that was never pressed, caused by current finding an unintended path through the matrix. Rollover is how many keys can be held at once. In everyday use both are reported as ghosting.
Does N-key rollover matter outside gaming?
Rarely. Typing seldom involves more than three simultaneous keys, and shortcut combinations use modifiers that usually sit outside the limit. It matters most for games that bind movement, sprint and an action to nearby keys.
Can a software update fix keyboard ghosting?
Not if it is a matrix limitation, because that is decided by the circuit board. Firmware can sometimes change the USB report mode from boot protocol to full report, which raises a six-key limit but cannot remove a collision.