Line focus

Line focus is the digital version of a card held under a line of print. A band of one, three, or five lines stays lit while the rest of the column drops back behind a dim wash, and the band moves down as the reader finishes each pass. Immersive Reader in Microsoft Edge popularised the feature with exactly those three widths, and the same idea turns up in ebook apps, dyslexia-focused readers, and a long tail of browser extensions.

It solves a specific failure, not a general one. Returning from the end of a line to the start of the next is the hardest movement in reading: the eye has to travel back across a whole measure and land one line lower, with nothing but line spacing to aim by. When that sweep misses, a reader either repeats the line just finished or skips the next one entirely, and usually notices only a sentence later. Tight leading, a long measure, and low contrast all make the miss more likely. A focus band removes the ambiguity by removing the alternatives: there is one place the eye can be. That helps readers with dyslexia and readers with attention difficulties most, and it helps anybody at all working through a wall of text on a phone at the end of a long day, which is the curb cut shape this feature has.

Two neighbours are easy to confuse with it. Read aloud also moves a highlight through text, but it is driven by speech: the voice sets the pace and the highlight follows the spoken word, so the reader is listening rather than tracking. Line focus makes no sound and the reader keeps the pace. Line clamp is the opposite operation altogether: it removes lines from the layout to make a block shorter, where line focus keeps every line and only changes which one is prominent. Nothing is hidden, so nothing has to be restored before a reader can search, select, or copy the passage.

The design cost is mostly in the plumbing. A band has to know where the lines are, which means it belongs to a reading surface with a predictable rhythm, not to arbitrary page content wrapped around floats and figures, and it has to survive dynamic type and reflow, where a rewrap changes both the line count and the line height. Advancing needs an input the reader can use without looking, so keyboard, tap, and scroll should all move the band. Make the wash a dimming rather than a blur, so a reader can still see where they are in the page, and let the band be turned off in one action: an aid that cannot be dismissed is a constraint.