Finger precision is not enough to hit a character gap
Aliases: caret placement · character gap · touch caret
What it is
A text caret has to sit in the gap between two characters. The gap has zero width, or only half a glyph of visual space, while a finger contact often covers several characters. A direct tap on the run lets the system guess the gap from the contact centroid; a wrong guess inserts at the neighbour. Finger precision fails here even when the letters are readable.
Why it happens
Hit-testing projects an areal contact onto a one-dimensional axis of glyph advances. Each gap’s acceptance region is about half a glyph to either side, narrower than a finger pad. A small centroid bias jumps the caret to the next gap. Leading, type size, and mixed CJK squares with Latin make acceptance regions swell and shrink, so the same finger’s success is unstable inside one paragraph. A desktop mouse hotspot is a pixel-level point that can rest on a gap; a finger has no such point. “Tap to move the caret” on touch is therefore a low-resolution quantisation, not an aim. The error shows up when editing starts: people think they tapped before the letter to change, and the new characters land after it.
Studying it
Have people place the caret before a specified character while varying type size, leading, typeface, and finger (thumb / index). Log first-hit, corrections, and contact offset from the target gap. Independent variables also include whether a second fine-adjustment is allowed. Split “landed in the neighbouring gap” from “landed on the neighbouring line.” A mouse control shows device difference; it is not the pass line for touch.
Where it stops holding
Very large type, or a field of only a few characters (a one-time code), can grow the gap’s acceptance region until it is enough. A stylus tip is an order of magnitude smaller; this claim weakens for pen. Once selection or arrow keys have parked the caret nearby, the problem becomes “how to nudge,” which is a different control.
Applying it
- Do not treat “tap the text” as the only precise caret; the first landing only has to reach the neighbourhood of a word.
- Body text at small sizes must offer a second placement (magnification, indirect sliding, or arrows) rather than demanding retaps until a lucky hit.
- Log character-distance from first landing to the target gap; frequent misses of more than one character mean the finger is still being asked to do pixel aiming it cannot do.