The magnifier solves occlusion, not precision
Aliases: caret loupe · occlusion preview · text magnifier
What it is
The magnifier that appears while dragging a caret or selection paints the sliver of text under the finger above the finger. It solves occlusion: the pad covers the gap to be seen, and without an offset view the loop cannot close. It does not turn the finger into a finer pointer—centroid jitter remains, and the caret can still only rest in those character gaps. Seeing and hitting are different purchases; the magnifier buys the former.
Why it happens
When the contact and the pixels to be seen coincide, the visual channel is blocked by the body, and the caret cannot be aligned with the target gap. The magnifier is an offset display: the sample sits under the contact, the image sits above it, with enough gap that the pad does not cover the image. The loop becomes “eyes on the image, finger in the sample.” Magnification makes glyphs readable and also magnifies centroid jitter: a millimetre of tremor jumps the caret further in the image, but the quantisation grid is still the original gaps, so docking precision does not rise. People who treat the magnifier as accuracy will keep nudging in the zoomed view and oscillate between two gaps. Offset techniques such as Shift are the same class: they reveal a covered point; they do not shrink the contact.
Studying it
With and without a magnifier, drag the caret to a specified gap and log time, overshoot, and whether people report “I saw the target.” Independent variables include offset direction, magnification, and whether hit-testing also changes. Split “saw the target but docked in the neighbour” (precision unchanged) from “could not see, so docked wrong” (occlusion). Scoring only final correctness mixes the two failures.
Where it stops holding
Pen and mouse do not have this occlusion; a magnifier is often surplus and even covers content. At very large type, occlusion eases and the magnifier may only block view. Too much magnification makes the image itself hard to locate on a line. A magnifier does not replace handle hit size and does not make the first tap more accurate.
Applying it
- Show an offset magnifier while dragging a caret or handle, defaulting above or beside the contact so the image is not covered by the same finger.
- Do not tune magnification as a precision knob; the factor only has to keep glyphs readable.
- In tests ask both “could you see the target gap?” and “which gap did you stop in?” Fail the first, change the magnifier; fail the second, change indirect nudging or type size.