The reading region must stay stable, not interrupted by floating controls

What it is

Mid-paragraph, a share button that follows the scroll slides into the line from the lower right and covers the rest of the sentence. The landing the eyes had just aimed at is gone; the line has to be found again. A stable reading region asks that the block holding the prose not be rewritten by floating controls while scrolling — no covering of letters, no stealing of the line, no moving under the reader’s gaze.

A button statically inserted after a paragraph is a different interruption. This is something that travels with gaze and rewrites those pixels in the moment of reading.

Why it happens

Reading is a string of short saccades along a line, with the next landing expected still on that stable line. A floating control that moves relative to the type inserts a moving occluder, or a new salient object, on the landing. After the saccade is cut, the line has to be re-aimed — closer to a lost-place search than to a normal inter-word hop. If the control also animates on, the motion transient pulls attention off word meaning, and the half-read sentence drops out of working memory.

The cost is highest for objects that depend on keeping place — long sentences, formulae, code — because once the landing is lost they are almost always reread from the start of the block.

Studying it

A reading task: the same article, one condition with a scroll-following floating button or a fading bottom bar, one without. Measure reading speed, regressions, and whether the covered line is reread.

Independent variables: whether the floating control enters the text column, whether its appearance is animated, whether it covers mid-line or end-of-paragraph. Dependent variables: reading speed, regression count, comprehension accuracy, time to re-aim the interrupted line.

Eye tracking sees the regressions cleanly. Without it, put a question on the line the floater covers that can only be answered from the hidden words.

Where it stops holding

A navigational feed is not a reading column; a floating price on a card is outside this claim. A video player’s floating scrubber operates a surface that is not for reading type. A locator button floating on a map is not in a reading region. A toolbar the user summoned (a tap on the screen) is an interruption they licensed, so long as the screen was stable before the tap.

Applying it

• Share, like, contents — park floating entries outside the text column, or on an edge that will not cover the current line while scrolling.
• Do not animate a control into the line currently being looked at during reading.
• System bars that must appear (a weak network, a download) enter from the top or bottom, not from the middle of the prose.
• How to check: scroll a representative article at the target type size, and see whether any control covers unread letters in any frame. One covering frame and the reading region is not yet stable.

Related

• Same group: F1.13.2 Buttons mixed into the reading flow cause misses and visual noise · F1.13.3 In long-form reading, park actions at the edge or on a scroll-triggered bar · F1.13.4 Separation is not absolute; inline actions such as text selection need an exception
• Nearby: F3.04 Scan patterns · F1.09 Overlay occlusion of the object
• Search terms: stable reading region · floating interrupt · regression saccade