The left-edge tap zone that a one-handed reader cannot reach
Tap zones look like the simplest feature in a reader: the left strip goes back, the right strip goes forward, the middle opens the menu. Then you use it on a phone, in one hand, on a bus, and discover that the geometry was designed for a device nobody holds that way.
The arithmetic
A typical phone reports a width of about 360 to 412 density-independent pixels. Take 360. A 20% edge zone is 72dp. At the standard density that is roughly 11 millimetres of glass — about the width of a thumbnail, measured from the bezel inward.
Now hold the phone in your right hand. The base of your thumb rests near the bottom-right corner and the thumb sweeps an arc whose centre is the bottom-right. The top-left corner is the furthest point from that arc: it is the one place on the screen you cannot reach without shifting your grip.
Put the “previous page” zone there and two things happen. Readers stop using it and swipe instead, or they shift their grip on every backward page turn, which is exactly the motion the zone was supposed to save.
Why the zones exist at all
Because swiping has its own failure mode. A horizontal swipe on a page of text competes with text selection, with horizontal scrolling in a zoomed PDF, and with the system’s own back gesture at the screen edge. A tap is unambiguous: it either hit a zone or it did not.
So the zones are worth keeping. The question is where they go and how wide they are.
What we changed
The default edge width is now narrower than it was, and the zones are mirrored vertically rather than spanning the full height.
The reasoning: on a phone, the reachable band is the lower two-thirds. A zone that spans the full height puts most of its area where the thumb is not, and — worse — puts part of it under the palm when the phone is held loosely. A zone that only occupies the lower band is smaller, easier to hit deliberately, and much harder to hit by accident.
The vertical band also resolves an old complaint about the top strip: on a phone with a notch or a punch-hole camera, the top edge is a system gesture area on some devices and the tap never reaches the app at all.
Accidental turns are worse than missed turns
A missed turn costs one swipe. An accidental turn costs the reader their place in a paragraph, and — if they do not notice immediately — a minute of confused re-reading.
So the zones are biased toward missing rather than firing: a tap has to land inside the band, not near its edge, and a tap that arrives within a few hundred milliseconds of a scroll is discarded rather than interpreted. That last rule exists because a scroll gesture that ends with a finger lift is, geometrically, a tap at the point where the finger stopped moving.
Why it is configurable
Because the correct answer depends on the hand, the device and the case. A 6-inch phone in a slim case and a 10-inch tablet in a folio are not the same instrument, and the same percentage produces an 11mm strip on one and a 40mm strip on the other.
The default is a guess that is right for most phones. The setting exists for the readers for whom the guess is wrong, and the widths are exposed in percentage terms rather than in millimetres because the app cannot know how wide the screen physically is.
The rule this produced
Any interaction that a reader performs more than a hundred times per book has to be evaluated on the geometry of the hand, not on the geometry of the layout. Page turns, brightness, and the menu gesture all qualify. The rest — the settings that are touched once — can be designed on a desktop monitor and nobody will notice.
What about the other hand
A left-handed reader holds the phone in the left hand and reaches the left edge easily. Every default that assumes a right hand is wrong for roughly one reader in ten, and it is wrong in a way that is invisible if you only test with your own hand.
The zones are therefore symmetric: the same band on both edges, mirrored vertically. The only asymmetry is which one turns forward, and that follows the reading direction rather than the hand — a left-to-right book turns forward on the right edge whether the reader is right- or left-handed.
For right-to-left scripts the mapping flips, which is the correct behaviour and a surprisingly common source of bugs in readers that hard-code “right means next”.
The double-tap trap
One more failure mode worth recording: the double tap.
In a two-page or zoomed view, a double tap means “zoom in”. In a single-page view it means nothing, so it falls through to whatever the single-tap handler does — which is a page turn. The result is that a reader who taps twice quickly, as people do when an interface feels slow, turns two pages and loses their place.
The fix is to hold the single-tap action for the duration of the double-tap window before committing it. It adds about 250 milliseconds of latency to every page turn, which sounds unacceptable and is not: the page turn is animated anyway, and the perceived delay is smaller than the annoyance of the alternative.
The next note is about text itself: why automatic hyphenation is off until you ask for it.