Key repeat

Hold a key down and it fires once, pauses, then fires over and over until you let go. The pause is the initial delay, the run after it is the repeat rate, and both numbers belong to the operating system rather than to any application: a typical desktop waits somewhere near half a second and then produces roughly thirty characters a second, and the reader may have moved either slider a long way from that. What reaches the page is a plain stream of keydown events with no gaps in it, one keyup at the very end, and a single flag, event.repeat, marking every event after the first.

That flag is the whole practical lesson. Anything cheap and incremental should be happy to be repeated, because repetition is exactly what the reader is asking for: moving a caret, deleting a character, nudging a selected object, stepping a value, scrolling a list. Anything expensive or irreversible must check the flag and refuse: a shortcut that opens a dialog, a key that submits, a handler that fires a request. Holding the key for a second is not a deliberate act repeated thirty times, it is one act with a slow finger on it, and code that cannot tell the difference will fire thirty dialogs. The same goes for counting: the number of keydown events is not the number of presses, so a press counter has to ignore repeats.

Repeat is also a place where people differ enormously. Operating systems let the delay be lengthened, the rate slowed, or repeat turned off entirely, and the accessibility settings around it (slow keys, which require a key to be held before it registers at all, bounce keys, which ignore a second press of the same key too soon after the first, and sticky keys) exist because a hand with a tremor, or one moving with a mouthstick, produces presses that a default configuration reads as either accidental or deliberate in exactly the wrong way. So an interface may lean on repeat as a convenience and must never depend on it: whatever a held key achieves needs to be achievable by pressing repeatedly, or better, by one press of something coarser.

That coarser thing is usually a modifier. The pattern in every design tool is a small step on the plain arrow and a large one with Shift held, which is where nudge lives, and it is what keeps repeat from being the only way to travel any distance. Repeat gives velocity; the modifier gives reach. The nice property of pairing them is that the reader never has to guess how long a hold will take to arrive somewhere, because the coarse step gets them close in a countable number of presses and the fine one finishes the job.